Docs: conceptual docs batch 1 (#27173)
Re-organizing some of the content involves runnables/lcel/streaming into
conceptual guides.
Conceptual guides added:
- [x] Runnables
- [x] LCEL
- [x] Chat Models
- [x] LLM
- [x] async
- [x] Messages
- [x] Chat History
- [x] Multimodality
- [x] Tokenization
Outstanding:
- [ ] Callbacks/Tracers
- [ ] Streaming
- [ ] Tool Creation
- [ ] Document Loading
Other conceptual guides are placeholders to make sure that no existing
links breaks.
Some high level re-organization:
* Introduce the Runnable interface prior to LCEL (since those are two
distinct concepts)
* Cross-link as much related content as possible (including how-to
guides)
