docs: Add build page; correct proxy info; fix Caddy example (#2579)

* Add build page; correct proxy info; fix Caddy example

* Improve Caddyfile example

* Apply review comments; add polylith Caddyfile

docs/installation/9_starting_monolith.md

@@ -0,0 +1,42 @@
+---
+title: Starting the monolith
+parent: Installation
+has_toc: true
+nav_order: 9
+permalink: /installation/start/monolith
+---
+
+# Starting the monolith
+
+Once you have completed all of the preparation and installation steps,
+you can start your Dendrite monolith deployment by starting the `dendrite-monolith-server`:
+
+```bash
+./dendrite-monolith-server -config /path/to/dendrite.yaml
+```
+
+By default, Dendrite will listen HTTP on port 8008. If you want to change the addresses
+or ports that Dendrite listens on, you can use the `-http-bind-address` and
+`-https-bind-address` command line arguments:
+
+```bash
+./dendrite-monolith-server -config /path/to/dendrite.yaml \
+    -http-bind-address 1.2.3.4:12345 \
+    -https-bind-address 1.2.3.4:54321
+```
+
+## Running under systemd
+
+A common deployment pattern is to run the monolith under systemd. For this, you
+will need to create a service unit file. An example service unit file is available
+in the [GitHub repository](https://github.com/matrix-org/dendrite/blob/main/docs/systemd/monolith-example.service).
+
+Once you have installed the service unit, you can notify systemd, enable and start
+the service:
+
+```bash
+systemctl daemon-reload
+systemctl enable dendrite
+systemctl start dendrite
+journalctl -fu dendrite
+```