chore: add M_Alvarez to CODEOWNERS
Context
Elliot requested that both @d.barrett and @M_Alvarez be added to CODEOWNERS so that subsequent MRs can be approved and merged without bottlenecks.
What's in this MR?
Adds @M_Alvarez to the * rule in CODEOWNERS and updates the header
comment to reference GitLab CODEOWNERS documentation.
Why
Unblocks merging of approved MRs that are waiting for a code owner approval:
- !350 (merged) refactor(v2/secret): remove deprecated NewClient constructor
- !349 (merged) docs(v2): add top-level README with architecture overview
- !347 (merged) docs(v2/httpserver): add README
- !348 (merged) docs(v2): add Example_ functions for pkg.go.dev discoverability