Reorg and polish HTTP Headers landing page

[SphinxKnight]

Description

After updating the page in French, I chose to go with an upstream reorganization for the HTTP Headers landing page as this currently:

• Displays a lot of experimental headers upfront: I moved them below (and did the same with non-standard/deprecated)
• Misses some experimental / deprecated inline macros (while the headers are tagged accordingly on the sidebar and on their respective pages): I added macros where relevant
• Lists some "experimental+deprecated" headers: I removed the links from the landing page as I don't believe exposing them here is helpful for this case.
• Contains various TBD useless as is blocks: I removed those and added a single Markdown comment in the contributing section.
• Links to some obsoleted RFCs (while corresponding MDN pages exist): fixed the links

Motivation

I hope this proposal

• Makes it clearer for beginners to know what's broadly available and what's not (looking at you Client Hints)
• Removes editorial cruft when work is tracked on Missing HTTP header-specific pages (known) #1458

Related issues and pull requests

#1458

[github-actions]

Preview URLs

• /en-US/docs/Web/HTTP/Headers

External URLs (3)

URL: /en-US/docs/Web/HTTP/Headers
Title: HTTP headers

• https://datatracker.ietf.org/doc/html/draft-ruellan-http-accept-push-policy-00 (2 times) (Note! This may be a new URL 👀)
• https://w3c.github.io/ServiceWorker/ (1 time) (Note! This may be a new URL 👀)
• https://wicg.github.io/webpackage/draft-yasskin-http-origin-signed-responses.html (3 times) (Note! This may be a new URL 👀)

[SphinxKnight]

I'm pondering whether X-Permitted-Cross-Domain-Policies and X-Powered-By should be deferred under Non-standard

[teoli2003]

I'm pondering whether X-Permitted-Cross-Domain-Policies and X-Powered-By should be deferred under Non-standard

This is debatable, but I like them there as they have security implications.

[teoli2003]

👍