Fix the broken API source links, and mark and cross-link the documentation trees

[m-reuter]

Every [source] link in the API documentation was a 404. doc/conf.py passed
_version_dict["git_branch"] to LinkCodeResolver, but that value is always empty:
_version_info() is called with the default sections="", and version.py only queries
the branch when sections is non-empty. So the links were built as
https://github.com/Deep-MI/FastSurfer/blob//CerebNet/apply_warp.py#L29-L53, note the
double slash, which GitHub answers with 404. Confirmed live on both published trees.

They now use the commit hash, which is populated, always resolves, and unlike a branch
keeps the line numbers matching the code that was documented. resolve_links.py rejects
an empty ref instead of silently producing blob//, and joins the path with exactly one
slash.

DOC_URL pointed at a page that does not exist. The OCI label baked into every
published image was https://fastsurfer.org/fastsurfer/dev, a 404, and
tools/Docker/build.py used https://deep-mi.org/fastsurfer/stable, also a 404 because
the path is case sensitive. Both now point at https://deep-mi.org/FastSurfer/..., as the
issue templates already did. pyproject.toml's documentation URL pointed at the research
landing page rather than the documentation.

The two published trees are now marked and cross-linked. A bar above the page content
says whether you are reading a release or the development version, and links to the other
tree. The condition uses GITHUB_REF_NAME, the same value doc.yml uses for
destination_dir, so the banner and the publish location cannot disagree.
doc/_static/doc-version-link.js resolves the sibling tree from rel="index" rather than
from a hardcoded path, because the bar is one string shared by every page and a relative
path would break on nested ones.

Verified by building with -WT --keep-going for both dev and stable, checking the
generated links from a nested page, and confirming the new link shape returns 200 against a
pushed commit.