Put all build-generated artefacts on built about cylc-doc

[oliver-sanders]

The documentation should be built into doc/<version>.

I have no idea why you are seeing things in build/sphinx, that directory doesn't exist for me before or after a build:

$ make clean html singlehtml
$ find . -name 'build'
$ find . -name 'sphinx'
$ find . -name '_build'

[kinow]

Oh, you are right, make clean html singlehtml gives me no build directory, only the doc folder. I must have had some old folder there. And nothing appears now in git status. Funny 😕

Only way to get the build directory now is python setup.py bdist/install/etc. Closing it, sorry!

[oliver-sanders]

No probs, probably an artifact of all the stuff I moved, good old git clean to the rescue!