[v6.2] docs: add ACME and other Setup steps + Setup FAQ

[inertial-frame]

This PR:

1. Addresses: Let's Encrypt Documentation unclear #7204
2. Provides additional instructions to assist with: TLS handschake error | acme can't get a cert for domain #6448
3. Support wider efforts surrounding: Auto-provision a signed TLS certificate for proxy with LetsEncrypt #4117
4. Moves the Installation Guide to the new Setup Section. (There are pending changes for that Guide so this should be merged or updated after those other items get merged.)
5. Addresses: master: teleport --insecure doesn't actually skip cert verification on startup #2040 with updates.

Two previous tickets are relevant:

1. Add acme to full YAML Reference #6716
2. Update docs to remove certbot step #6696

I've reviewed most of our existing ACME documentation and it's pretty scant, covers how to automatically acquire a cert through the command line and addressed 443 but not in a consistent way. Ideally, we'd be refactoring all of those items into this one as well but for now, we'll likely leave them in place (we now have a target destination for anything about ACME if we decide to simply redirect/link people to this article - open to that as well).

Also, we mention that we recommend using subdomains in a few places but don't go into much detail about this. The comments above apply to this as well. Ideally all "setup" (read: first time or very common installation steps would be moved in here too).

[klizhentas]

The structure of the changes looks good, but let's make a couple of tweaks.