[docs] Made Advanced its own section (#1883)

* [docs] Made Advanced its own section

This splits the Advanced page off from the Getting Started guide and
makes it its own thing. It now has some additional sub-sections for
bigger topics like caching and enhanced security. This also moves
tracing from Getting Started to Advanced as that feels like a more
appropriate location for it.

The enhanced security looks a little silly with a single section, but I
have guides pending for firewall configurations and I'd also like to
consolidate our how to provision TLS certificates in there as we repeat
this information multiple times.

* [docs] Fix all my spelling errors

* [docs] Inline the links in sandboxing

docs/advanced/outgoing-proxy.md

@@ -0,0 +1,21 @@
+# Outgoing HTTP proxy
+
+GoToSocial supports canonical environment variables for configuring the use of an HTTP proxy for outgoing requets:
+
+* `HTTP_PROXY`
+* `HTTPS_PROXY`
+* `NO_PROXY`
+
+The lowercase versions of these environment variables are also recognised. `HTTPS_PROXY` takes precedence over `HTTP_PROXY` for https requests.
+
+The environment values may be either a complete URL or a `host[:port]`, in which case the "http" scheme is assumed. The schemes "http", "https", and "socks5" are supported.
+
+## systemd
+
+When running with systemd, you can add the necessary environment variables using the `Environment` option in the `Service` section.
+
+How to do so is documented in the [`systemd.exec` manual](https://www.freedesktop.org/software/systemd/man/systemd.exec.html#Environment).
+
+## Container runtime
+
+Environment variables can be set in the compose file under the `environment` key. You can also pass them on the CLI to Docker or Podman's `run` command with `-e KEY=VALUE` or `--env KEY=VALUE`.