Remove local builds of the docs from the repository

[Ed McDonagh]

Currently each time a change to the documentation is tested, a whole load of docs build files change and need to be committed.

[Ed McDonagh]

Removed the docs _build files from the repository and added the content to .gitignore so they won't get readded. Fixes #77.

→ <<cset 77ab08162a3d>>

[Ed McDonagh]

Updated changes. Refs #77.

→ <<cset 771c38876598>>