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/development/feature_documentation/README.md

@@ -0,0 +1,12 @@
+# Feature Documentation
+
+This section covers implementation documentation for various features of Synapse.
+
+If a developer is planning to make a change to a feature of Synapse, it can be useful for
+general documentation of how that feature is implemented to be available. This saves the
+developer time in place of needing to understand how the feature works by reading the
+code.
+
+Documentation that would be more useful for the perspective of a sysadmin, rather than
+a developer who's intended to change to code, should instead be placed in
+[Usage](../../usage/).