Add stable public Python API (mkdocs.build / mkdocs.serve) - #76

Summary

Implements stable public Python API** (upstream mkdocs#1240 "API for running mkdocs"): a stable, documented, programmatic API for building and serving docs — no more from mkdocs.__main__ import build_command hacks or subprocess calls.

What changed

• mkdocs.build() — programmatic equivalent of mkdocs build:
  • Loads the config from a file path, file-like object, or stdin (-), with Path accepted
  • Any config option can be overridden via keyword arguments (site_name, nav, strict, ...)
  • Fires plugin on_startup/on_shutdown events exactly like the CLI
  • Returns the loaded MkDocsConfig (e.g. config.site_dir)
• mkdocs.serve() — programmatic equivalent of mkdocs serve (blocks until shutdown), with the same override semantics
• CLI parity: mkdocs build / mkdocs serve commands now call the same public functions, so CLI and API can never drift apart
• Widened commands.serve.serve()'s config_file annotation to str | IO | None (the CLI already passes file objects at runtime)
• Docs: new Python API user-guide page (build/serve examples, programmatic config overrides, no-config-file pattern) + API reference entries in dev-guide/api.md (mkdocstrings)
• Tests: mkdocs/tests/api_tests.py (build end-to-end incl. Path, file objects, kwargs overrides, dirty mode; serve argument forwarding); updated CLI serve tests for the normalized watch list

Verification

• uvx prek run -a passes (incl. mypy)
• Full test suite: 753 tests OK
• Docs site builds cleanly; mkdocstrings renders both signatures with annotations
• py.typed already ships with the package, so the new API is covered by PEP 561 type declarations

Examples

import mkdocs

config = mkdocs.build(config_file="mkdocs.yml", site_name="My project")
print(f"Built site to {config.site_dir}")