msk-operations: fix MkDocs strict link and shorten SKILL.md description

- README.md: replace relative CloudFormation template link with absolute
  GitHub URL so the strict MkDocs build resolves it.
- SKILL.md: shorten frontmatter description to ~935 chars (under the 1,024
  limit) while retaining primary MSK activation terms and exclusions
  (MSK Connect / Serverless / Replicator, IaC authoring, other services).