Set edit links dynamically for RtD preview and PR builds

[mnocon]

Target: 5.0, 6.0

The current link checker has a bug: for new Markdown files, it reports 404 for "View on GitHub" button:

#3279 (comment)

Errors in site/release_notes/cohesivo_v6.0_deprecations/index.html

    [404] https://github.com/ibexa/documentation-developer/blob/5.0/docs/release_notes/cohesivo_v6.0_deprecations.md (at 194:10) | Rejected status code: 404 Not Found

This is because the 5.0 branch is hardcoded as "edit_uri", and it looks into the 5.0 branch for changes that were not merged yet.

This PR sets the value dynamically, so for it will work correctly for PRs and for ReadTheDocs preview builds.

Doc:

• GITHUB_HEAD_REF : https://docs.github.com/en/actions/reference/workflows-and-actions/variables
• ReadTheDocs does not have access to the branch name:

The [slug](https://docs.readthedocs.com/platform/stable/glossary.html#term-slug) of the version being built, such as latest, stable, or a branch name like feature-1234. For [pull request builds](https://docs.readthedocs.com/platform/stable/pull-requests.html), the value will be the pull request number.

(from https://docs.readthedocs.com/platform/stable/reference/environment-variables.html )

So for RTD previews, commit hash is used.

Tested locally by setting the variable before building and checking the output in the site dir:

export READTHEDOCS_VERSION_NAME=marek
~/python/bin/python3.13 -m mkdocs build --strict
cat site/index.html| grep marek
<a href="https://github.com/ibexa/documentation-developer/blob/marek/docs/index.md" title="source.link.title" class="source-github">

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