Ensure, that sphinx works

This is a trivial intro message, just to make sure docs can be written
and built.

May serve as a reference commit on how to add new pages to
documentation.

docs/index.rst

@@ -10,6 +10,8 @@ Welcome to Birdvisu's documentation!
    :maxdepth: 2
    :caption: Contents:
 
+   intro
+
 
 
 Indices and tables

docs/intro.rst

@@ -0,0 +1,9 @@
+Introduction
+@@@@@@@@@@@@
+
+Birdvisu is a library and a graphical app to visualize routing topologies of
+OSPF. It does not implement OSPF on its own, but rather exports current state
+from `BIRD`_ – that's where the name comes from.
+
+
+.. _BIRD: https://bird.network.cz