Specify doctree location in Sphinx build

[ericholscher]

Otherwise it defaults to being in the output,
and gets copied to the webheads,
bloating disk usage.

[gregmuellegger]

I actually used this "feature" when working annotatedocs. I scraped the zips of all current sphinx builds. They contained the doctree files that I was able to parse and had then a AST available of the docs without going through the trouble of building all the docs locally :) Was really useful to test the project on many existing projects.

This is just an anecdote I wanted to let you know. Absolutely no argument against this change :)

[ericholscher]

Ha, yea. They are taking up like ~50G+ of disk space on our servers though, which is less cool.