Push built docs to GitHub pages branch

[dstansby]

In #132 the doc build was fixed on GitHub actions, but the built docs aren't uploaded anywhere. To do to fix this issue:

• Upload the built docs to a GitHub pages branch
• Change any references to readthedocs to the new doc URL

[samcunliffe]

Partially reached by #172 (#166) and #173 .
But currently not showing (where it's apparently deployed) at https://matplotlib.org/napari-matplotlib

[dstansby]

Just emailed a Matplotlib org admin to see what we can do to fix this.