[24299] Add fallback branch for master links (backport #1241)

[mergify]

Description

Fix GitHub links in the documentation to always point to the correct branch instead of defaulting master/main

get_git_branch() now reads READTHEDOCS_VERSION on RTD builds (for branch and tag builds) instead of relying on git name-rev, which returned weird names like "external-1234". There are 3 possible cases for RTD builds now:

• Branch builds -> Github links refer to the branch itself
• Tag builds -> Github links refer to the tag itself
• PR preview builds (VERSION_TYPE=external) now return None and fall back to the default branch as it seemed impossible to get the real branch name.

@Mergifyio backport 3.5.x 3.4.x 3.2.x 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.
• 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.

---

This is an automatic backport of pull request #1241 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 60e9c7d has failed:

On branch mergify/bp/3.2.x/pr-1241
Your branch is up to date with 'origin/3.2.x'.

You are currently cherry-picking commit 60e9c7d.
  (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)

Changes to be committed:
	modified:   docs/fastdds/discovery/discovery_server.rst
	modified:   docs/fastdds/discovery/static.rst
	modified:   docs/fastdds/getting_started/simple_app/includes/sum_and_next_steps.rst
	modified:   docs/fastdds/getting_started/simple_python_app/includes/sum_and_next_steps.rst
	modified:   docs/fastdds/rtps_layer/rtps_layer.rst
	modified:   docs/fastdds/security/access_control_plugin/access_control_plugin.rst
	modified:   docs/fastdds/transport/shared_memory/shared_memory.rst
	modified:   docs/fastdds/transport/tcp/tcp.rst
	modified:   docs/fastdds/use_cases/request_reply/request_reply.rst
	modified:   docs/fastdds/use_cases/statistics_module/statistics_module.rst
	modified:   docs/fastdds/use_cases/well_known_deployments/well_known_deployments.rst
	modified:   docs/fastdds/use_cases/zero_copy/zero_copy.rst
	modified:   docs/fastdds/xml_configuration/making_xml_profiles.rst
	modified:   docs/fastddsgen/pubsub_app/includes/sum_and_next_steps.rst
	modified:   docs/installation/sources/sources_linux.rst
	modified:   docs/installation/sources/sources_mac.rst
	modified:   docs/installation/sources/sources_windows.rst
	modified:   docs/notes/migration_guide.rst
	modified:   docs/notes/notes.rst
	modified:   docs/notes/previous_versions/v2.6.10.rst
	modified:   docs/notes/previous_versions/v2.6.11.rst
	modified:   docs/notes/previous_versions/v2.6.9.rst
	modified:   docs/notes/previous_versions/v3.0.0.rst
	modified:   docs/notes/versions.rst

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both modified:   docs/conf.py
	deleted by us:   docs/fastddsgen/rpc_calculator_basic_app/includes/app.rst
	deleted by us:   docs/fastddsgen/rpc_calculator_basic_app/intro.rst
	deleted by us:   docs/fastddsgen/rpc_calculator_feed_app/includes/app.rst
	deleted by us:   docs/fastddsgen/rpc_calculator_feed_app/intro.rst
	both modified:   docs/fastddsgen/usage/usage.rst
	both modified:   docs/installation/configuration/cmake_options.rst

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