Ada manuals structure

[peer35]

Update the structure of the Ada manuals to get them more in line with the others. I still don't like that what is now the 'about' page duplicates the topic page, but I also see the sense in keeping it all together for better navigation. We should revisit the structure of the manuals section and tools topics later this year.

@Kostusas, can you live with this for now?

Other changes:

• Fixed links to the Yoda manuals.
• Moved the sidebar configuration to /manuals/ada/_sidebar.yml.

Closes #597

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[peer35]

Since there is a "documentation" project we are going to review and possibly renew the manuals section anyway. I propose we leave it as is for now and discuss any new ideas with Kostas and Wiebe.