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/handle_errors.md

@@ -26,7 +26,7 @@ handle_errors {
 
 Custom error pages based on the status code (i.e. a page called `404.html` for 404 errors):
 
-```caddy
+```caddy-d
 handle_errors {
 	rewrite * /{http.error.status_code}.html
 	file_server
@@ -35,7 +35,7 @@ handle_errors {
 
 A single error page that uses [`templates`](/docs/caddyfile/directives/templates) to write a custom error message:
 
-```caddy
+```caddy-d
 handle_errors {
 	rewrite * /error.html
 	templates
@@ -43,9 +43,9 @@ handle_errors {
 }
 ```
 
-Reverse proxy to a professional server that is highly qualified for handling HTTP errors and improving your day:
+Reverse proxy to a professional server that is highly qualified for handling HTTP errors and improving your day 😸:
 
-```caddy
+```caddy-d
 handle_errors {
 	rewrite * /{http.error.status_code}
 	reverse_proxy https://http.cat {
@@ -56,8 +56,19 @@ handle_errors {
 
 Simply use [`respond`](/docs/caddyfile/directives/respond) to return the error code and name
 
-```caddy
+```caddy-d
 handle_errors {
 	respond "{http.error.status_code} {http.error.status_text}"
 }
 ```
+
+To handle specific error codes differently, use an [`expression`](/docs/caddyfile/matchers#expression) matcher:
+
+```caddy-d
+handle_errors {
+	@4xx expression `{http.error.status_code} >= 400 && {http.error.status_code} < 500`
+	respond @4xx "It's a 4xx error!"
+
+	respond "It's not a 4xx error."
+}
+```