fix: update GTR Day 0 section based on CNCF TOC review feedback

Address feedback raised by joshgav in the CNCF TOC technical review:
cncf/toc#2173 (comment)

- Scope: clarify target persona, add ManagedClusterSet intra-org isolation note
- Usability: add APIs/CRDs interaction point, Helm, update UI to reference Headlamp
- Design: add pull-based model as explicit design principle, rewrite service
  dependencies, IAM, sovereignty, compliance, release process sections
- Installation: add addon framework description
- Security: fix Secure Artifacts sub-bullet formatting
- General: bold question prompts, fix broken links, typos, indentation, CRD casing

Signed-off-by: august simonelli <asimonel@redhat.com>

Author: augustrh