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/command-line.md

@@ -222,7 +222,8 @@ NOTE: Due to [a bug in Go](https://github.com/golang/go/issues/29228), version i
 <pre><code class="cmd bash">caddy reload
 	[--config &lt;path&gt;]
 	[--adapter &lt;name&gt;]
-	[--address &lt;interface&gt;]</code></pre>
+	[--address &lt;interface&gt;]
+	[--force]</code></pre>
 
 Gives the running Caddy instance a new configuration. This has the same effect as POSTing a document to the [/load endpoint](/docs/api#post-load), but this command is convenient for simple workflows revolving around config files. Compared to the `stop`, `start`, and `run` commands, this single command is the correct, semantic way to change/reload the running configuration.
 
@@ -234,6 +235,8 @@ Because this command uses the API, the admin endpoint must not be disabled.
 
 `--address` needs to be used if the admin endpoint is not listening on the default address and if it is different from the address in the provided config file. Note that only TCP addresses are supported at this time.
 
+`--force` will cause a reload to happen even if the specified config is the same as what Caddy is already running. Can be useful to force Caddy to reprovision its modules, which can have side-effects, for example: reloading manually-loaded TLS certificates.
+
 
 
 ### `caddy reverse-proxy`