Manually adding version 0.18.1 with Sphinx build.

[dhermes]

Also adding it as stable/ since that is the latest tag.

Done via:

$ git worktree add -b add-docs-0.18.1 ../hotfix official/gh-pages
$ git checkout 0.18.1
$ tox -e docs
$ cd ../hotfix
$ rm -fr stable
$ git rm -fr stable
$ mkdir stable
$ mkdir 0.18.1
$ cd -
$ cp -R docs/_build/html/ ../hotfix/stable/
$ cp -R docs/_build/html/ ../hotfix/0.18.1/
$ cd ../hotfix
$ git add stable/
$ git add 0.18.1/

[tseaver]

LGTM