doc: Fix orphaned documentation pages for Features

[stephane-caron]

This PR fixes the Features page. There are two sides to the update:

• All pages doc/a-features/*.md that were in parallel and empty at the top level are now subpages of Features
• The Features page becomes an actual TOC

Before:

After:

Look at the menu to the left as well:

This is one step towards fixing #2408 but it does not address the whole issue yet.

[hrp2-14]

Hi ! This project doesn't usually accept pull requests on the main branch.
If this wasn't intentionnal, you can change the base branch of this PR to devel
(No need to close it for that). Best, a bot.

[github-actions]

👋 Hi,
This is a reminder message to assign an extra build label to this Pull Request if needed.
By default, this PR will be build with minimal build options (URDF support and Python bindings)
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Thanks.
The Pinocchio development team.

[jcarpent]

Thanks a lot for the quick fix