On the version selector for the documentation

[janmenjayap]

This PR includes the code for the version selector for the documentation.

Added a new HTML code to display the Version: #
Added the relevant versions in doc/en/website/conf.py.

This PR introduces the flexibility to switch between multiple versions in the documentation.

Fixes #37384.

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes. (Not required it seems.)
• I have updated the documentation accordingly. (Not required it seems.)

⌛ Dependencies

None

[janmenjayap]

Hello.
I have some questions.

1. How to know the version of the documentation that is currently opened (for e.g. -- 'current_version': "10.2", but instead of 10.2 it can be 10.3-beta or any local version; how to check that)?
2. Do I need to creat a separate versions.html file or changes in home.html is fair enough?
3. Do I need to change any other conf.py file including these versions?

It will be absolutely helpful if these questions are answered.

Thank you :-)

[mkoeppe]

I've merged the current development version to trigger the build of the documentation in the CI.

[mkoeppe]

1. How to know the version of the documentation that is currently opened (for e.g. -- 'current_version': "10.2", but instead of 10.2 it can be 10.3-beta or any local version; how to check that)?

The Sphinx configuration in src/sage_docbuild/conf.py obtains this version information from sage.version.version

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