docs: add comprehensive codebase summary

[liyin2015]

What does this PR do?

Adds a CODEBASE_SUMMARY.md document providing a comprehensive overview of the AdalFlow repository. This serves as a quick-start guide for new contributors and developers to understand the project structure and architecture.

Summary covers:

• Project overview -- AdalFlow as a PyTorch-style library for building and auto-optimizing LLM workflows
• Tech stack -- Python 3.9-3.14, Poetry, core and optional dependencies (LLM providers, vector DBs)
• Repository structure -- Full directory tree with descriptions of each module
• Architecture -- Core abstractions (Component, GradComponent, Generator, Parameter, DataClass, ModelClient, Retriever), call interface patterns, optimization pipeline, agent system, data/storage layers, and tracing
• Component flow diagram -- Visual overview of how Runner, Agent, Generator, ModelClient, Retriever, and Tracing connect
• Build and CI -- Makefile targets and GitHub Actions workflows (testing, docs, release)
• Public API -- Key exports and import conventions
• Configuration -- Environment setup, component config system, and YAML configs

No breaking changes.

Before submitting

• Was this discussed/agreed via a GitHub issue? (not for typos and docs)
• Did you read the contributor guideline?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests? (not for typos and docs)
• Did you verify new and existing tests pass locally with your changes?
• Did you list all the breaking changes introduced by this pull request?