PRO-1312: Document the automated CLA process

[qunabu]

Context

Documents the Contributor License Agreement process, which until now pointed contributors at a Google Form with no explanation attached.

The contributing guide asked contributors to sign a CLA, but nothing explained why it is required, what it covers, where signature records live, or how a signature is verified before a pull request is merged. PRO-1312 asks for that explanation in one place, for HyperFormula and Handsontable alike.

Two facts in particular were not written down anywhere a contributor would look:

• The CLA is now enforced automatically. A GitHub App sets a required cla/signed status check on every pull request, so an unsigned PR cannot be merged and no reviewer has to check anything by hand.
• One signature covers both projects. The record is keyed to a GitHub account, not to a repository, so somebody who signed for Handsontable is already cleared here — and the other way round.

What changed:

• docs/guide/contributing.md — new Contributor License Agreement section covering the reasoning, the cross-project scope, the four steps a contributor sees, the required check, where records are stored, and a note for reviewers. Step 1 of "How to get started" now links to it.
• .github/pull_request_template.md, README.md, docs/index.md — the retired Google Form link is replaced by the signing page.

The signing page and the full process description are served by the CLA gate worker, which lives in a separate repository. The Handsontable counterpart to this PR is linked below; both repositories deliberately carry the same wording.

How did you test your changes?

Documentation-only change; no library code is touched.

• Verified no Google Form reference survives outside generated VuePress output (docs/.vuepress/dist/, which is rebuilt):

  grep -rn "goo.gl/forms" --include="*.md" . | grep -v node_modules | grep -v ".vuepress/dist"

• Confirmed the in-page anchor #contributor-license-agreement matches the new heading, so step 1 of "How to get started" resolves both on GitHub and on the rendered docs site.
• Confirmed both linked pages return HTTP 200 and render the expected content.

Types of changes

• Breaking change (a fix or a feature because of which an existing functionality doesn't work as expected anymore)
• New feature or improvement (a non-breaking change that adds functionality)
• Bug fix (a non-breaking change that fixes an issue)
• Additional language file, or a change to an existing language file (translations)
• Change to the documentation

Related issues:

1. PRO-1312
2. Handsontable counterpart: PRO-1312: Document the automated CLA process handsontable#13175

Checklist:

• I have reviewed the guidelines about Contributing to HyperFormula and I confirm that my code follows the code style of this project.
• I have signed the Contributor License Agreement — one signature covers both HyperFormula and Handsontable; the cla/signed check on this PR confirms it.
• My change is compliant with the OpenDocument standard.
• My change is compatible with Microsoft Excel.
• My change is compatible with Google Sheets.
• I described my changes in the CHANGELOG.md file.
• My changes require a documentation update.
• My changes require a migration guide.

(The OpenDocument/Excel/Sheets and changelog items do not apply to a documentation-only change.)

Before merging

Two operational steps must land first, or contributors get a worse experience than today:

1. Import the historical signatures into the gate's database, so people who signed the Google Form are not asked to sign again. An importer and instructions ship in the CLA gate repository.
2. Add cla/signed as a required check on develop, so it actually blocks a merge. The App is now installed here and the check has been verified green on this PR.

ClickUp task: https://app.clickup.com/t/9015210959/PRO-1312

---

Note

Low Risk
Documentation-only changes with no runtime or library code impact.

Overview
Replaces the retired Google Form CLA link with cla.handsontable.com/sign in the PR template, README.md, and docs/index.md, and clarifies that one signature covers HyperFormula and Handsontable via the cla/signed check.

Adds a Contributor License Agreement section to docs/guide/contributing.md (why the CLA exists, cross-repo scope, PR flow with the GitHub App, required check, EU D1 storage, reviewer guidance). Step 1 of “How to get started” now links to that section instead of the old form URL.

^{Reviewed by Cursor Bugbot for commit 25c96da. Bugbot is set up for automated code reviews on this repo. Configure here.}

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[github-actions]

Performance comparison of head (96bf2f2) vs base (61ead73)

                                     testName |    base |    head | change
--------------------------------------------------------------------------
                                      Sheet A |  499.34 |  514.39 | +3.01%
                                      Sheet B |  163.02 |  168.59 | +3.42%
                                      Sheet T |  139.22 |  149.69 | +7.52%
                                Column ranges |  476.91 |  486.79 | +2.07%
                                Sorted lookup | 14330.5 | 15139.4 | +5.64%
Sheet A:  change value, add/remove row/column |   16.99 |   17.83 | +4.94%
 Sheet B: change value, add/remove row/column |  152.37 |  151.73 | -0.42%
                   Column ranges - add column |  162.81 |  161.79 | -0.63%
                Column ranges - without batch |  500.65 |  480.87 | -3.95%
                        Column ranges - batch |  126.71 |  120.76 | -4.70%

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.31%. Comparing base (61ead73) to head (96bf2f2).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1725   +/-   ##
========================================
  Coverage    97.31%   97.31%           
========================================
  Files          195      195           
  Lines        15719    15719           
  Branches      3455     3455           
========================================
  Hits         15297    15297           
  Misses         414      414           
  Partials         8        8           

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