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 #2171 (comment)), 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. kubernetes/website#49416 (comment)).

• 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 Upgrade Docsy to 0.6 kubernetes/website#49416, I'm ok waiting

Related:

• [CI/infra] Move optional NPM package dependencies to dev #2115
• [CI] Docsy NPM-package: global install fails as of 0.8.0 #2122
• Use Hugo as a peerDependency instead of hiding it #2171