[9.5 branch ] Add .github/workflows/auto_tag_stable.yml

[rouault]

This workflow automatically updates and pushes the 'stable' tag when commits are pushed to the branch pointed by /~https://github.com/OSGeo/PROJ/settings/variables/actions/STABLE_BRANCH

Workflow experimented in a toy repository in /~https://github.com/rouault/try_autotag/actions

[rouault]

Once this is merged, we need to update the RtD configuration:

• in https://readthedocs.org/dashboard/osgeo-proj/redirects/, modify /* -> /en/9.5/:splat to be /* -> /en/stable/:splat
• in https://readthedocs.org/dashboard/osgeo-proj/edit/, modify the default version to be "stable"

[rouault]

First run successful: /~https://github.com/OSGeo/PROJ/actions/runs/11890393496/job/33128939801 has created /~https://github.com/OSGeo/PROJ/tree/stable

[rouault]

That required additional fine tuning with 25e7cbd and 1ddaee4 because otherwise the "stable" tag and "9.5" branch were exactly the same commit, and apparently that confused RTD that only triggered the build for the "9.5" branch and not the "stable tag". Now we add a dummy commit to the stable tag, and RTD seems happy now and automatically triggers a stable build: https://readthedocs.org/projects/osgeo-proj/builds/

[rouault]

ok, "stable" RTD build went fine, and 2 configuration items of #4323 (comment) now updated. We should be all good now ! (CC @jjimenezshaw )

[jjimenezshaw]

Thank you @rouault !