Create sections for navigation menu

[azeey]

🎉 New feature

Summary

Currently, the navigation menu on gazebosim.org/docs is a flat list and can be difficult to navigate. There also no clear organization between items that are tutorials vs how-to guildes vs reference, etc. This PR organizes the documents based on the diataxis framework.

To make all of this possible, the build script had to be modifed.

1. Navigation menu is now only defined in the index.yaml files in the versioned directories (e.g. jetty/index.yaml). The top level index.yaml no longer contains pages. This allows for more control over the order of items and organizing them into different sections.
2. Since the pages are now in the versioned directories, files that are common to all gazebo versions have been moved to the common directory. This is to make it explicit in the index.yaml files where these files are coming from. Otherwise, there it would be confusing since the common files are no longer peers of the index.yaml files that reference them. When referencing these files in index.yaml, the prefix common: has to be used now.
3. The build script adds functionality to add sections.
4. Pages without a file, but with children will be considered organizational elements and the appropriate file for sphinx is automatically generated.

TODO

• Sections are only defined for jetty (so, only build_multiversion.py -r jetty works right now)
• Define User and Developer personas
• Update README

Preview

Checklist

• Signed all commits for DCO
• Updated documentation (as needed)
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