docs: add standalone reference page for @hydra.main() decorator

[dhruvildarji]

Summary

Closes #845.

This PR adds website/docs/advanced/hydra_main.md, a dedicated reference page for the @hydra.main() decorator.

The page covers:

• What @hydra.main() does — a plain-English description of the full runtime lifecycle (argument parsing, Hydra initialization, config composition, HydraConfig population, and task function invocation).
• All parameters with type, default, and detailed description:
  • config_path — relative paths, pkg:// prefix, None, and how the default interacts with version_base.
  • config_name — how Hydra resolves the primary config file; what None means.
  • version_base — a table of supported values and their behavioral differences, with a recommendation to always set it explicitly.
• Return value behavior — including the multirun note.
• HydraConfig global state — how to access job name, output directory, and overrides from inside the task function; when HydraConfig.get() raises.
• Common usage patterns — minimal app, app with config file, app with Structured Config, and a multirun sweep example.
• Cross-references to the Compose API, decorating_main, and version_base upgrade guide.

The new page is registered in website/sidebars.js under Reference manual, immediately before the existing Compose API entry.

Test plan

• Verify website/docs/advanced/hydra_main.md renders correctly in the Docusaurus dev server (cd website && npm start).
• Confirm the new entry appears in the sidebar under Reference manual.
• Check all internal links (compose_api.md, decorating_main.md, ../upgrades/version_base.md, search_path.md) resolve without 404s.
• Review parameter descriptions against hydra/main.py for accuracy.

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