fix hanging docs build + bad cleanup logic

[balopat]

fix hanging docs build:

• -j auto speeds up the sphinx build but in some cases causes the Github Actions build to hung
• I added a flag fast to the build_rtd_docs script to make it conditional

fix cleanup logic in rtd docs build (this only comes up when you try to build things locally twice):

• the cleanup script was removing a non-exsistant directory, I fixed it to point to ./docs

[dstrain115]

Could you add a more descriptive PR comment when you submit (similar to the comments in the PR itself should be sufficient)?

[balopat]

You're right I should have updated it - it would have been nicer to have a more substantial commit message - I just haven't read your comment in time, and couldn't stop Cirqbot merging (we should have an undo label?:D) I added the description at least it will be in the PR description historically - I don't want to force update to master to amend the commit msg.