Adds API reference documentation generated from docstrings and improves the documentation build process.

Changes include:

• API Reference Docs: Generated API reference documentation for the following modules: engine, vectorstore, loader, and history. This makes it easier for users to understand and use the library's classes and methods.
• Documentation Testing: Integrated documentation testing into the CI workflow using Nox. This ensures that documentation builds successfully and catches any issues early.
• HTML Templates & Symlinks: Added HTML templates and symlinks for the README and changelog to streamline the documentation build.
• Fixed Doc Build Warnings: Resolved warnings related to the README, engine and document_store that were appearing in the docfx build log. This improves the overall quality and reliability of the documentation build.

b/367372730