docs: Some cleanup, some v2.4.0 additions (#139)

* docs: Some cleanup, some v2.4.0 additions

- Add `abort` directive docs
- Add a note in `handle` to cross-link to `handle_path`
- Add another example in `handle_errors` that shows how an `expression` matcher can be used to pair with it
- Add a cat emoji to handle_errors because 😸
- Add `file_server` to one of the `php_fastcgi` examples, it's rare that you'll ever use it without `file_server` so might as well include it there
- Add a TOC to `reverse_proxy` cause it's such a long page. Maybe we'll add one to other pages as well, but TBD
- Clarify the upstream address stuff a bit after some discussion in https://caddy.community/t/reverse-proxy-with-multiple-different-upstreams-with-paths/11512/12 and mention `rewrite` as the alternative
- Use the `{re.*.*}` shortcut in the Caddyfile matcher docs, links to the placeholder mapping to let people trace where that comes from

* docs: Revise the `handle` cross-linking, add another example

* docs: Bump minimum Go version to 1.15

* docs: A bunch more additions since v2.3.0

I went through the whole list of commits here: https://github.com/caddyserver/caddy/compare/v2.3.0...ec3ac84

* docs: Review feedback

* docs: Link to https://golang.org/doc/install, better instructions

src/docs/markdown/caddyfile/options.md

@@ -49,6 +49,7 @@ Possible options are:
 	}
 	key_type ed25519|p256|p384|rsa2048|rsa4096
 	cert_issuer <name> ...
+	ocsp_stapling off
 
 	# Server Options
 	servers [<listener_address>] {
@@ -134,7 +135,10 @@ Configures [On-Demand TLS](/docs/automatic-https#on-demand-tls) where it is enab
 Specifies the type of key to generate for TLS certificates; only change this if you have a specific need to customize it.
 
 ##### `cert_issuer`
-Defines the issuer (or source) of TLS certificates. The tokens following the name of the issuer set up the issuer the same as if specified in the [`tls` directive](/docs/caddyfile/directives/tls#issuer).
+Defines the issuer (or source) of TLS certificates. The tokens following the name of the issuer set up the issuer the same as if specified in the [`tls` directive](/docs/caddyfile/directives/tls#issuer). May be repeated if you wish to configure more than one issuer to try. They will be tried in the order they are defined.
+
+##### `ocsp_stapling`
+Can be set to `off` to disable OCSP stapling. Useful in environments where responders are not reachable due to firewalls.