* [docs] Add a certificates guide in Advanced
This adds some documentation about the process of getting certificates
through ACME in general. It also provides a number of links to
alternative clients and certbot deployment guides that are up to date.
Slightly restructure the NGINX and Apache reverse proxy documentation
and insert mentions to the Provisioning TLS certificates advanced
documentation in them.
* [docs] Add firewall section in Advanced
* [docs] Add new guides to section indexes
* [docs] Fix spelling issue
* [docs] Fix a few typos
