Docsy as an NPM package: publish package, adjust dev deps, and document choices

[chalin]

We've faced issues with use of Docsy as an NPM dependency because we currently only support github package specifiers of the form github:google/docsy#semver:0.6.0. When performing an install (be it install or ci), NPM will always "dev" build the Docsy github packages (as I mention in https://github.com/google/docsy/issues/2171#issuecomment-2588443164), and hence fetch all of Docsy's dev and peer dependencies. This isn't a bug, it is a consequence of how NPM works and should be documented.

As we upgraded Kubernetes.io we faced a problem because hugo-extended (a dev dependency) couldn't be fully installed. Thankfully, @SayakMukhopadhyay identified the problem and proposed a solution: which is to include gcompat in the Docker image build (cf. https://github.com/kubernetes/website/pull/49416#discussion_r1913913975).

• [ ] I'm content atm leaving Docsy NPM usage via github access, but we should document the consequences detailed above.
• [ ] Soon(ish), I'd like to consider actually publishing Docsy to https://www.npmjs.com, but given the solution proposed in https://github.com/kubernetes/website/pull/49416, I'm ok waiting

Related:

• #2115
• #2122
• #2171