Restructure documentation (#2114)

Breaks up the ARC documentation into several smaller articles. 

`@vijay-train` and `@martin389` put together the plan for this update, and I've just followed it here. 

In these updates:

- The README has been updated to include more general project information, and link to each new article.
- The `detailed-docs.md` file has been broken up into multiple articles, and then deleted.
- The Actions Runner Controller Overview doc has been renamed to `about-arc.md`.

Any edits to content beyond generally renaming headers or fixing typos is out of scope for this PR, but will be made in the future. 

Co-authored-by: Bassem Dghaidi <568794+Link-@users.noreply.github.com>

docs/monitoring-and-troubleshooting.md

@@ -0,0 +1,30 @@
+# Monitoring and troubleshooting
+
+## Metrics
+
+The controller also exposes Prometheus metrics on a `/metrics` endpoint. By default this is on port `8443` behind an RBAC proxy.
+
+If needed, the proxy can be disabled in the `values.yml` file:
+
+```diff
+metrics:
+  serviceAnnotations: {}
+  serviceMonitor: false
+  serviceMonitorLabels: {}
++ port: 8080
+  proxy:
++   enabled: false
+```
+
+If Prometheus is available inside the cluster, then add some `podAnnotations` to begin scraping the metrics:
+
+```diff
+podAnnotations:
++ prometheus.io/scrape: "true"
++ prometheus.io/path: /metrics
++ prometheus.io/port: "8080"
+```
+
+## Troubleshooting
+
+See [troubleshooting guide](../TROUBLESHOOTING.md) for solutions to various problems people have run into consistently.