Fix "Edit Page" button visibility on stable branches

[daksh1403]

Replaces the hardcoded source_branch = "master" in conf.py with dynamic branch detection.

Rationale The "Edit on GitHub" button currently links to master regardless of the active branch. This causes broken links (404s) or hides the button entirely when viewing documentation for stable release branches (e.g., release-1.13) where file paths or content may differ.

Implementation Details

The source_branch is now resolved in the following order:

Read the Docs: Checks os.environ["READTHEDOCS_GIT_IDENTIFIER"].

Local Dev: Falls back to git rev-parse --abbrev-ref HEAD.

Fallback: Defaults to "master" on error.

Fixes #17947

[sterliakov]

Release branches do not receive any updates, they are carved in stone, so why should "edit page" link to branches other than master? User-visible 404 errors are bad, but receiving PRs against release branches for a file that no longer exists would be no less awkward.

Hiding this button altogether for non-master docs sounds reasonable to me, but using other target branches does not.

[wyattscarpenter]

You've accidentally included many irrelevant changes in this PR, from your other PRs.

[wyattscarpenter]

It seems to me (from testing on my fork) that this also will result in 404s once we remove the stale release branches and rely entirely on tags as suggested in #19795. Since this seems to use the tag name to send to a github url that would correspond to a branch (and not a tag).

So, I would recommend just closing this. And, independently closing #17947 as complete.