refactor: migrate docs from Sphinx/RST to MkDocs/Markdown

[wuqunfei]

Replaced the Sphinx-based documentation system with MkDocs for easier maintenance and better ReadTheDocs integration.

Changes:

• Delete all RST documentation files
• Create new Markdown documentation with MkDocs Material theme
• Add mkdocs.yml configuration
• Update .readthedocs.yaml for MkDocs builds
• Update Makefile with new doc commands (7-doc-build, 7-doc-serve, 7-doc-deploy)
• Update pyproject.toml with MkDocs dependencies

New documentation structure:

• docs/index.md - Home page
• docs/quickstart.md - Quick start guide
• docs/installation.md - Installation guide
• docs/configuration.md - Configuration guide
• docs/async-guide.md - Async usage guide
• docs/streaming.md - Streaming guide
• docs/authentication.md - Authentication guide
• docs/api/ - API reference (functions, response, client, exceptions)
• docs/examples/ - Basic and advanced examples
• docs/contributing.md - Contributing guide
• docs/changelog.md - Changelog