Tutorials Grouped By Section + Best Practices Broken Up

[sabine]

This patch does two things:

1. Group tutorials by section ("Get Started", "Language", "Platform Tools", "Guides") so they can show up under the respective tab
2. Break up the "Best Practices" page into several "Platform Tools" tutorials.

As part of (1), we remove the HTMX navigation from the Learn area, since the sidebars do not scroll enough to warrant the complexity now.

This brings the Learn Homepage into a state where a layout according to Figma and the documentation rework can be implemented.

The tabs, for now, lead to the first tutorial in that section, except for the Platform/Exercises/Books pages, which have their own dedicated page.

page	screenshot
Learn Homepage
Get Started
Language
Platform Tools
Guides

[tmattio]

Thanks @sabine, that looks very good! I do have comments on the overall organisation, but since the documentation outline is still in progress, I don't think it's helpful to discuss this as part of this PR.
The only thing I feel semi-strongly about is the PPX guide under the Platform tool. Ppxlib is a platform tool, but PPX is a language concept. The guide would be better placed under language in my opinion.
This shouldn't block the merge, so I'm approving nonetheless and leaving it to you to decide what's best.

[sabine]

I'll move the PPX guide to language.