Remove redundant Makefile target
Add chaining as a glossary term
[Fix #339] Document chaining of syntax checkers

Extract the :next-checkers property of syntax checkers and create an
admonition which documents the syntax checker chain for each checker.
Add a pseudoxml target to dump the doctrees

Makes debugging of our custom extensions easier.
Use the default theme

The new theme on ReadTheDocs is really much nicer than the built-in
Sphinx theme we used now.
Add Go errcheck checker