docs: add skills-only path to getting started tutorial
Restructures the getting started page so users who only want skills
can follow steps 1-3, hit a path fork, and jump straight to a
self-contained skills section — no git repo or beacon needed.
Three edits:
- Prerequisites now note git is only needed for caps
- Path fork after step 3 routes skills-only users to #skills
- Skills section rewritten as standalone entry point (browse → vet → learn)
Note: run `make deploy-site` to publish these changes.