Remove RST docs

[danepowell]

These are the source files for the documentation that lives on https://docs.acquia.com/blt/

It's a pain to keep them in sync with the docs repository, and also makes it difficult to contribute since you have to submit changes to BLT first, then sync to docs, then await editorial review, and finally sync editorial changes back to BLT.

Realistically, 90% of the people who contribute to BLT docs are Acquians who can already contribute directly to the docs team, and these source files aren't human-readable anyway, so I don't see any reason to keep them here.