Remove ability to conditionalize parts of the documentation

[tobiasdiez]

The PR #36495 introduced the ability to disable building parts of the documentation based on some features. This was meant to help the modularization project, but was never actually used.

On the other hand, it checks for every of the around 200 document the presence of every feature, leading to quite a considerable overhead for building the documentation. Thus this feature is reverted for now (until it's actually needed and perhaps can be re-implemented in a better way).

📝 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.
• I have updated the documentation and checked the documentation preview.

⌛ Dependencies

[github-actions]

Documentation preview for this PR (built with commit c687717; changes) is ready! 🎉
This preview will update shortly after each push to this PR.

[tobiasdiez]

Failing tests seem to be false-alerts.

@dimpase could you please have a look at this PR. Thanks!

[dimpase]

thanks for taking care of this

[vbraun]

The new chinese tour also needs the same treatment, can you add it after the next beta is released?

#39792

[fisheryv]

The new chinese tour also needs the same treatment, can you add it after the next beta is released?

#39792

Of course, I will follow this work.