The special sha is no longer needed, let's bump to a recent version of
hugo instead and that should make it all work nicely (for a while).
Updating hugo requires changing to hugo.toml and forcing mermaid to be
enabled, without those changes the build failed with
`failed to extract shortcode: template for shortcode "mermaid" not found`
This reverts commit 5909717700
Part-of: <https://gitlab.freedesktop.org/libinput/libei/-/merge_requests/358>
Since we want the generated documentation to be useful on-disk, drop the
baseURL and switch to relativeURLs = true for hugo.
This is arguably a bit nasty in that it git clones the theme into the
build directory. But oh well...
