Update to Docsy v0.9.1

[aykevl]

I'm doing this to stay up to date, and because I'm looking forward to dark mode support in Docsy v0.10 (which according to https://github.com/google/docsy/issues/331 seems to be almost finished).

There are a few small visual changes, which I think are generally an improvement:

• Buttons are more square.
• Links now have underscores, for accessibility reasons.
• The ellipsis after "Read more..." on the home page is gone.

I did a quick browse through the website and couldn't find any breaking changes.

Note that not all hugo versions seem to work. My Fedora-shipped version v0.111.3 resulted in errors, and the latest version also results in errors for me. However v0.122.0 works for me.

[aykevl]

Huh, it also fails in Netlify. Probably because of the Hugo version requirement.

[aykevl]

Let's do this some other day when these issues are hopefully fixed.

[aykevl]

See #407 for version 0.10.0.