docs(flows): document where @persist stores its SQLite database

[Beandon13]

Summary

• Issue [FEATURE] Enhance the document about @persisit #5372 asks where on disk @persist actually saves state. The Flow Persistence section in concepts/flows.mdx mentions SQLite but never names the file or its path.
• Adds a new "Where Is the State Stored?" subsection that:
  • Names the default file (flow_states.db) and explains it's resolved through appdirs.user_data_dir.
  • Tabulates the per-platform defaults for macOS / Linux / Windows (matching how appdirs 1.4 ignores appauthor on POSIX but uses it on Windows — verified locally).
  • Notes that the project segment is cwd's basename by default and how to pin it with CREWAI_STORAGE_DIR.
  • Shows runtime introspection via crewai.utilities.paths.db_storage_path().
  • Documents how to override the path entirely with SQLiteFlowPersistence(db_path=...).
• Pure docs change, no code touched.

Closes #5372

Testing

• Verified path resolution against appdirs.user_data_dir and crewai.utilities.paths.db_storage_path():

python -c "from crewai.utilities.paths import db_storage_path; print(db_storage_path())"
# /Users/<me>/Library/Application Support/<cwd-basename>
CREWAI_STORAGE_DIR=my-flows python -c "from crewai.utilities.paths import db_storage_path; print(db_storage_path())"
# /Users/<me>/Library/Application Support/my-flows
python -c "from crewai.flow.persistence.sqlite import SQLiteFlowPersistence; p=SQLiteFlowPersistence(); print(p.db_path)"
# /Users/<me>/Library/Application Support/<cwd-basename>/flow_states.db

• No build infra is shipped in the repo (Mintlify renders remotely), so I left rendered preview to the docs CI.