chore: recursion howto explainers

[signorecello]

Description

This PR also moves things around to accomodate one new how-to and its correspondent explainer

Problem*

An explainer and a guide on recursion was long overdue, but very much needed.

Summary*

It may seem like a lot of files but most of them are just things moving around:

• Created a new top-level category "Noir" to avoid mixing syntax stuff with high-level explanations. That's most of the file moves
• Added a divider on the sidebar
• In the new "explanation" category, I wrote an explainer. This is one of the two files that needs to be reviewed
• In the how to category, I wrote a guide. This also needs to be reviewed
• I found out the links to @site were valid... But they route to the assets, thus rendering the page in markdown format 😱 The docusaurus documentation is not entirely clear about that, and I clearly missed the tiny reference to that fact in the advanced pages.

[github-actions]

🚀 Deployed on https://657c71e602230b26ac6079f0--noir-docs.netlify.app

[TomAFrench]

I'm not following some of the examples you've given. Could you explain a bit more as it's not clear to me why we need to do so many proofs.

Also, I think the recursion interface is going to change soon so technical docs may be out of date soon.

[TomAFrench]

Technical stuff looks ok now. I think that we're not being totally clear about why you would use different architectures of recursive proofs however.

[signorecello]

Thanks @TomAFrench just made some changes as suggested. Lmk if there's anything else, otherwise let's merge please because there's a bug in master so this needs to go in 0.21.0

[critesjosh]

Thanks for adding the caveats at the top of the How To Guide. That page can still use some improvement, but I think it's ok to ship now and we can iterate on it based on feedback we get. 🚀