Move code docs to a readthedocs project.

Ported the existing docs, but the real goal is getting the docs for
campaign version moved here, as well as the manual. RTD will have an
"edit on github" button that'll keep this accessible to players, but
it'll be much easier to keep docs up to date while developing features
if it's part of the code base.

docs/dev/index.rst

@@ -0,0 +1,8 @@
+Developer documentation
+=======================
+
+.. toctree::
+   :maxdepth: 2
+   :caption: Contents:
+
+   design/index.rst