docs(community-management): add Session, Matrix, and SimpleX guides

[AllyPry]

Summary

Adds security documentation for three privacy-focused messaging platforms — Session, Matrix, and SimpleX — following
the two-layer structure already used for Discord, Telegram, and X.

Closes #599.

What this adds

Account management guides (procedure pages, modelled on signal.mdx):

Page	Focus
guides/account-management/session.mdx	Recovery password custody, privacy toggles, Account ID verification, Community server limits
guides/account-management/matrix.mdx	Recovery key setup, cross-signing, room configuration, federated moderation, homeserver operations
guides/account-management/simplex.mdx	Database passphrase, SimpleX Lock, encrypted backups, connection hygiene, group roles

Community management hubs (framework pages, modelled on community-management/telegram.mdx):

Page	Focus
community-management/session.mdx	Threat model, risk table, topic index into the Session guide
community-management/matrix.mdx	Threat model, risk table, topic index into the Matrix guide
community-management/simplex.mdx	Threat model, risk table, topic index into the SimpleX guide

Navigation and supporting changes:

• vocs.config.ts: three entries under Community Management, three under Guides → Account Management (alphabetical
  position preserved)
• Page maps updated in community-management/overview.mdx and guides/account-management/overview.mdx
• Regenerated index.mdx for both touched folders
• wordlist.txt: added Draupnir, Mjolnir, Orbot, SOGS, XFTP
• Fixed a pre-existing MD022 markdownlint failure in guides/account-management/overview.mdx (missing blank line
  between two adjacent headings)

Why the platform-specific angle

Each platform is covered on its own terms rather than as a variation on Signal, because the failure modes differ:

• Session removes phone numbers and central accounts, which also removes recovery and revocation. The recovery
  password is the account, and Communities running on a Session Open Group Server are not end-to-end encrypted.
• Matrix is federated, so room membership is a data-distribution decision: every homeserver with a member in a room
  holds a copy of it. Encryption is per room and per device, which makes cross-signing an operational control rather
  than a nicety.
• SimpleX has no user identifiers at all. The invitation link is the access control and the local database is the
  identity, so backup discipline and link rotation replace the account-recovery and ban primitives operators expect.

Sourcing

Settings paths and platform behaviours were taken from vendor documentation rather than from memory, and each page
links its sources under Further reading: Session Docs and the Session security blog, Element documentation and the
Matrix.org moderation guide, and the SimpleX privacy, app settings, and groups guides. Where client UI labels differ
between platforms or releases (notably Element's Security & Privacy → Encryption rename), the pages say so and
point to the vendor documentation for current paths.

Validation

• cspell — 27 files checked, 0 issues
• markdownlint-cli2 — 0 issues
• validate:content — 7/7 pages in community-management, 18/18 in guides/account-management
• docs:build — clean, 887 files generated
• Internal links and heading anchors verified against the built HTML; every cross-link between hub and guide resolves

[github-actions]

Sidebar Configuration Reminder

This PR includes added, renamed, or removed documentation files:

• docs/pages/community-management/matrix.mdx (added)
• docs/pages/community-management/session.mdx (added)
• docs/pages/community-management/simplex.mdx (added)
• docs/pages/guides/account-management/matrix.mdx (added)
• docs/pages/guides/account-management/session.mdx (added)
• docs/pages/guides/account-management/simplex.mdx (added)

Please ensure that:

• The sidebar in vocs.config.ts has been updated to include these files
• New content has the dev: true parameter so it's marked as under development
• Sidebar links match the file paths - use the preview deployment to verify

See Contributing Guide – Sidebar & Navigation for more details.

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_{This is an automated reminder. If this PR doesn't need sidebar changes, you can ignore this message.}