3di TOC restructure (backport #6898)

[ahcorde]

Backport of #6898 to the lyrical branch, squashing all 40 commits of the PR into a single commit.

Description

Manually backport #6898

Did you use Generative AI?

Claude Opus 4.7

[github-actions]

HTML artifacts: https://github.com/ros2/ros2_documentation/actions/runs/30103237599/artifacts/8600760101.

To view the resulting site:

1. Click on the above link to download the artifacts archive
2. Extract it
3. Open html-artifacts-6998/index.html in your favorite browser

[ahcorde]

@kscottz I backported this to lyrical as requested in the original PR, do you mind to take a look. This is a big change

[kscottz]

Just finally getting a chance to look at this. Am I fair to assume this is even with rolling?

[kscottz]

Woof. There are a lot of things that could be better here. There are a lot of articles that seem really out of place or shoe horned into categories. I started trying to call them out but that doesn't seem particularly useful.

I don't think forward porting from Lyrical is the right the answer here. The logical solution would be to just force this through and fix the issues on rolling. @fujitatomoya @ahcorde what are your thoughts here?

[RachelPotts]

@kscottz About the articles that seem out of place: please do let us know about them, and we'll address quickly. For info, we're already working through this to-do list (some relate to our previous discussions, where we've now also had input from Geoff too). These will start coming though as individual PRs soon.