Docs for Siren (#4023)

This adds some documentation for the Siren app into the Lighthouse book.

Co-authored-by: Mavrik <[email protected]>

Author: AgeManning

.gitignore

@@ -12,3 +12,4 @@ genesis.ssz
 
 # IntelliJ
 /*.iml
+.idea

book/src/SUMMARY.md

@@ -33,6 +33,11 @@
         * [Authorization Header](./api-vc-auth-header.md)
         * [Signature Header](./api-vc-sig-header.md)
     * [Prometheus Metrics](./advanced_metrics.md)
+* [Lighthouse UI (Siren)](./lighthouse-ui.md)
+	* [Installation](./ui-installation.md)
+	* [Configuration](./ui-configuration.md)
+	* [Usage](./ui-usage.md)
+	* [FAQs](./ui-faqs.md)
 * [Advanced Usage](./advanced.md)
     * [Checkpoint Sync](./checkpoint-sync.md)
     * [Custom Data Directories](./advanced-datadir.md)