semantic-versionify version number for non-releases

`1.0.0-dev` means development version leading up to 1.0.0.
move doc files to .rst so they render on github
update indexes to use .rst, remove .txt refs
fix api docs post txt -> rst rename
Merge pull request #3802 from ivanov/doc-to-rst

This renames our documentation files to .rst so that it renders nicely on GitHub

The docs build just fine with these changes, it'd be good to get this in before any more changes are made to the docs
Merge pull request #3800 from minrk/semver

semantic-versionify version number for non-releases