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.md

@@ -23,6 +23,14 @@ handle [<matcher>] {
 
 If you prefer crafting HTTP handler logic in a more inheritence-based way like nginx location blocks, you may prefer the use of `handle` blocks rather than defining mutually-exclusive matchers for your directives. If inheritence is a desired characteristic of your HTTP handler configurations, then the `handle` directive may suit you well.
 
+## Similar directives
+
+There are other directives that can wrap HTTP handler directives, but each has its use depending on the behavior you want to convey:
+
+- [`handle_path`](handle_path) does the same as `handle`, but it strips a prefix from the request before running its handlers.
+- [`handle_errors`](handle_errors) is like `handle`, but is only invoked when Caddy encounters an error during request handling.
+- [`route`](route) wraps other directives like `handle` does, but with two distinctions: 1) route blocks are not mutually exclusive to each other, and 2) directives within a route are not [re-ordered]([directive order](/docs/caddyfile/directives#directive-order), giving you more control if needed.
+
 ## Examples
 
 Handle requests in `/foo/` by the static file server, and send all other requests to the reverse proxy:
@@ -35,3 +43,15 @@ handle {
 	reverse_proxy 127.0.0.1:8080
 }
 ```
+
+You can mix `handle` and [`handle_path`](handle_path) directives in the same site, and they will still be mutually exclusive from each other:
+
+```caddy-d
+handle_path /foo/* {
+	# The path has the "/foo" prefix stripped
+}
+
+handle /bar/* {
+	# The path still retains "/bar"
+}
+```