Changes to remove documentation build warnings.

--HG--
rename : docs/database.rst => docs/database.txt
rename : docs/depgraph.rst => docs/depgraph.txt
rename : docs/dist.rst => docs/dist.txt
rename : docs/metadata.rst => docs/metadata.txt
rename : docs/util.rst => docs/util.txt
rename : docs/version.rst => docs/version.txt