feat: use native Satteri admonitions

[MohamedH1998]

Summary

• replace source-level admonition rewriting with a scoped native Satteri AST plugin
• preserve plain titles, aliases, nested content, code examples, and per-file opt-outs
• require compatible explicit processors or admonitions: false
• align native APIs with Satteri 0.10 while isolating source-position parsing on an explicit 0.9 alias
• add upgrade guidance for explicit processors and admonitions in .md files

Validation

• pnpm --filter nimbus-docs test (1,652 passed, 1 skipped)
• focused native/adapter regressions (109 passed)
• pnpm --filter nimbus-docs typecheck
• pnpm lint
• BREAKING_CHANGE=1 pnpm upgrades:check
• pnpm api-reference:check
• pnpm templates:check
• clean final review

Stacked on #138. Split from and intended to replace the native-admonition portion of #131. The Workers registry feasibility path remains externally blocked by a Cloudflare registry 401.

[pkg-pr-new]

pnpm add https://pkg.pr.new/@cloudflare/create-nimbus-docs@139

pnpm add https://pkg.pr.new/@cloudflare/nimbus-docs@139

commit: 5244e11