refactor!: fully switch to Diataxis system

[ahal]

BREAKING CHANGE: This will break pretty much every link to our docs.

Awhile back we agreed to try and implement the Diataxis documentation system: https://diataxis.fr/

However, we only moved a handful of docs over and have been in a mixed state of having docs following two separate structures (if you can call the old way a structure) ever since.

The goal of this PR is to re-organize all the docs in the old structured over to the new Diataxis-based structure. There are plenty of docs that don't fit neatly in one place or the other, so this PR often takes a best guess at where it belongs.

This PR also doesn't split up any individual files, though it does sometimes move docs from directories to different places.

Rendered

[gbrownmozilla]

lgtm - thanks!

I think we should make a pass at updating links in all the places we can think of.

We should discuss the changelog; I'm leaning toward retiring it - I haven't found it very useful, and we haven't been keeping it up to date.

[gabrielBusta]

Some redirects that might be nice to have:

• https://docs.mozilla-releng.net/en/latest/procedures/release-duty/
• https://moz-releng-docs.readthedocs.io/en/latest/taskcluster/ci_admin.html
• https://moz-releng-docs.readthedocs.io/en/latest/how-to/release/mozillavpn.html

[ahal]

(fixed merge conflicts)