Allow checkout to tags in 2.14.x (backport #1278)

[mergify]

Description

This PR brings the modification to conf.py introduced in #1277 to enable build of ReadTheDocs documentation pointing to tags instead of branches. It also updates the fallback branches to their specific development branch and brings up the feature to checkout to tags.

@Mergifyio backport 2.14.x

Contributor Checklist

• Commit messages follow the project guidelines.
• Code snippets related to the added documentation have been provided.
• Documentation tests pass locally.
• N/A: The Pro version badge has been added if the documented feature is exclusive to Fast DDS Pro.
• Applicable backports have been included in the description.

Reviewer Checklist

• The PR has a milestone assigned.
• The title and description correctly express the PR's purpose.
• Check contributor checklist is correct.
• CI passes without warnings or errors.

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This is an automatic backport of pull request #1278 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 537ce61 has failed:

On branch mergify/bp/2.14.x/pr-1278
Your branch is up to date with 'origin/2.14.x'.

You are currently cherry-picking commit 537ce61.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   docs/conf.py

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[mergify]

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