Add some section pages

These pages simply act as a container for other pages, but also serve to provide
an introduction and explanation for the pages to come. We can also use it to
describe what topics should be covered in the section, to help others organise
new documentation pages.

docs/setup/README.md

@@ -0,0 +1,3 @@
+# Setup
+
+In this section, you will learn how to install and configure your own Synapse homeserver.

docs/setup/installation.md

@@ -0,0 +1,7 @@
+<!--
+  Include the contents of INSTALL.md from the project root without moving it, which may
+  break links around the internet. Additionally, note that SUMMARY.md is unable to 
+  directly link to content outside of the docs/ directory. So we use this file as a 
+  redirection.
+-->
+{{#include ../../INSTALL.md}}