docs: render experimental API notices

[Soumoditya]

Summary

• preserve the @experimental marker while AutoAPI parses source files
• render a stability warning for marked functions, methods, and properties

Fixes #4300.

Validation

• exercised the patched parser with bare and parenthesized decorators, plus marked properties and methods
• rendered every affected template with marked and unmarked objects
• passed pre-commit.ci - pr

AI assistance

This change was developed with substantial assistance from OpenAI Codex. I reviewed the AutoAPI parser contract, the repository's existing documentation configuration, and the resulting diff and focused validation output before submitting it.