chore: use rspack for faster builds

[tenstad]

https://docusaurus.io/blog/releases/3.6#docusaurus-faster

Experimental but safe
Don't be afraid to turn this feature on. What's experimental is the config options.

The new infrastructure is robust and well-tested by our CI pipeline. The Docusaurus site already uses it in production, and we plan to use it on other Meta docs sites as well.

What changed?

Started using faster for faster builcs.

webpack kept as dep due to

➤ YN0002: │ weave-gitops-docs@workspace:. doesn't provide webpack (pd0468), requested by file-loader and other dependencies.

Why was this change made?
To reduce build time from 6m to 2m20s

https://github.com/weaveworks/weave-gitops/actions/runs/12546069919/job/34981281244
https://github.com/weaveworks/weave-gitops/actions/runs/12546164995/job/34981533607

How was this change implemented?

How did you validate the change?

Release notes

Documentation Changes