Move concepts into a separate directory

Starting to structure the scaffold for the concepts. Moving concept
content into their own pages.

TBD what we'll end up doing with the actual concepts page in terms of
visual layout.

Update architecture page

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)

Co-authored-by: Eugene Yurtsev <[email protected]>

Add tools conceptual page

* Fix some typos
* Add some missing links

Streaming concept docs

Collecting some feedback

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Co-authored-by: Lance Martin <[email protected]>

Consolidate the docs. After this, we'll need to potentially set up
redirect, and update links to existing llm pages.

Still not sure this is a good idea. May be better to rename the concept
page and redirect appropriately better, since we do still have string in
/ string out llm integrations

Update architecture page

* update concepts index page
* update callbacks
* add document loaders
* update architecture to not use links in section headings

… concept_docs

Co-authored-by: Harrison Chase <[email protected]>

Add a few concepts that were missing

Add more missing concepts

Adding more concepts

Conversation patterns

Make it a bit more provider agnostic