Merge branch 'summary' into 'develop'

Add a Gitbook-ish SUMMARY.md

See merge request soapbox-pub/rebased!234

docs/README.md

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+# Rebased
+
+**Rebased** is a Fediverse backend written in Elixir.
+It's compatible with the Mastodon API and is the recommended backend for Soapbox.

docs/SUMMARY.md

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+# Summary
+
+* [Introduction](README.md)
+
+* Administration
+  * [Backing up](administration/backup.md)
+  * [Updating](administration/updating.md)
+
+## Configuration
+
+* [Cheatsheet](configuration/cheatsheet.md)
+* [Hardening](configuration/hardening.md)
+
+* Guides
+  * [Setting up the media proxy](configuration/howto_proxy.md)
+  * [Saving config in the database](configuration/howto_database_config.md)