* chore(ci): upgraded linter to 1.53.3
This flagged a bunch of unused parameters, so the PR is larger than
usual, but 99% mechanical.
* separate lint CI task
* run Lint in separate CI
* chore(site): upgraded site to latest hugo and removed vendored files
This switches to using github.com/google/docsy as a Hugo module,
so we don't need to vendor the theme anymore or its dependencies.
* layout
* more style, edit MD support
* style
* style
* fixed linter errors
* update text
