doc: Remove 'invocation' document

This contains no useful information at this point. Better to simply drop
it.

Signed-off-by: Stephen Finucane <stephen@that.guru>

doc/man/index.rst

@@ -0,0 +1,21 @@
+Man Pages
+=========
+
+These are the applications provided as part of Sphinx.
+
+Core Applications
+-----------------
+
+.. toctree::
+   :maxdepth: 3
+
+   sphinx-quickstart
+   sphinx-build
+
+Additional Applications
+-----------------------
+
+.. toctree::
+   :maxdepth: 3
+
+   sphinx-apidoc