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/directives/php_fastcgi.md

@@ -97,9 +97,10 @@ When using php-fpm listening via a unix socket:
 php_fastcgi unix//run/php/php7.4-fpm.sock
 ```
 
-The [`root` directive](/docs/caddyfile/directives/root) is often used to specify the directory containing the PHP scripts:
+The [`root` directive](/docs/caddyfile/directives/root) is often used to specify the directory containing the PHP scripts, and the [`file_server` directive](/docs/caddyfile/directives/file_server) to serve static files:
 
 ```caddy-d
 root * /var/www/html
 php_fastcgi 127.0.0.1:9000
+file_server
 ```