Motivation

• Provide a consolidated set of documentation for OCC covering API, design, usage, installation, pending issues, and a codebase audit.
• Normalize fragmented docs and make onboarding and maintenance easier for users and contributors.
• Offer a developer-facing reference to clarify architecture, extension points, and ranking/filtering behavior.

Description

• Added documentation files under doc/: doc/api-reference.org, doc/design-full.org, doc/usage.org, doc/installation.org, doc/pending.org, and doc/codebase-audit.org.
• Updated README.org to include a Documentation section linking to the new doc/ files.
• New content covers API entry points and variables, end-to-end architecture and object model, ranking and filtering design, installation and usage examples, pending issues/roadmap, and audit findings.
• This is a documentation-only change and does not modify Emacs Lisp runtime behavior or source code logic.

Testing

• No automated tests were executed for this documentation-only change.
• The existing automated test suite (ERT) was not modified and remains unaffected by these additions.

Codex Task