Add link to documentation page in our package metadata

[jherland]

Our project page on PyPI, https://pypi.org/project/fawltydeps/, has two links to our GitHub repo ("Homepage" and "Repository"), but no links to our new documentation page at https://tweag.github.io/FawltyDeps/.

That should be fixed. Also consider if there are more places we should link the new documentation page. Maybe there's a suitable shield/badge we can put at the top of the README?