docs: `graphs` update (#19675)
Issue: The `graph` code was moved into the `community` package a long
ago. But the related documentation is still in the
[use_cases](https://python.langchain.com/docs/use_cases/graph/integrations/diffbot_graphtransformer)
section and not in the `integrations`.
Changes:
- moved the `use_cases/graph/integrations` notebooks into the
`integrations/graphs`
- renamed files and changed titles to follow the consistent format
- redirected old page URLs to new URLs in `vercel.json` and in several
other pages
- added descriptions and links when necessary
- formatted into the consistent format
