docs: Integrate with readthedocs.org

This patch adds configuration to integrate with readthedocs.org, using
mkdocs as rendering engine.

It also does minor documentation updates, to tidy some things up
(clearer titles, move some documentation from the root into docs/, etc).

.readthedocs.yml

@@ -0,0 +1,8 @@
+# Read the Docs configuration file
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details.
+version: 2
+
+mkdocs:
+  configuration: .mkdocs.yml
+
+