docs: generate and publish API documentation

[prateek-dagar]

This PR configures Sphinx documentation for the langcodes repository to resolve issue rspeer#41 and adds the configuration for Read the Docs.

(Note: The issue was originally opened in the rspeer/langcodes repository, but since that repo is not actively maintained and new releases are published from this active fork, I am raising the PR here instead.)

Key Changes:

• Switch theme to modern 'sphinx-book-theme'.
• Configure MyST-Parser to render the README.md directly on the main index page.
• Exclude internal submodules (such as build_data, data_dicts, language_lists, registry_parser, tag_parser, util) to present a clean, public-facing API reference.
• Add sphinx-copybutton for code block convenience.
• Add 'docs' test environment to tox.ini for local validation.
• Configure .readthedocs.yaml using the latest stable ubuntu-24.04 image with Python 3.12 for automated Read the Docs builds.
• Integrate the documentation build test check into the GitHub Actions CI workflow.