docs: add pydoc-markdown doc-generation scripts for Python API reference

[twishabansal]

Summary

Adds the doc-generation scripts that turn the SDK source into the Hugo site PR #688 scaffolds.

Commits are split so copy and edit never mix. Commit 1 copies the scripts directly from the Go SDK repo. Following commits perform edits.

Testing

Built each package as its own site root (the production model) and served the static tree:

BASE=http://localhost:8000/
pip install -r docs-site/requirements.txt
rm -rf docs-site/public
bash scripts/generate-api-docs.sh core       dev "$BASE"
bash scripts/generate-api-docs.sh adk        dev "$BASE"
bash scripts/generate-api-docs.sh langchain  dev "$BASE"
bash scripts/generate-api-docs.sh llamaindex dev "$BASE"
bash scripts/generate-root.sh "$BASE"
cd docs-site/public && python3 -m http.server 8000

On the pinned toolchain (Hugo 0.152.2 extended + pydoc-markdown 4.8.2), manually verified at localhost:8000.