Created basic stuff document runnable creation function

[wpfl-dbt]

Context

We want to summarise but don't have the router to do it yet. This PR adds a basic stuff document runnable with unit tests that check basic functionality.

Changes proposed in this pull request

• Add format.py and runnables.py as new files in the core API
• Add our first runnable factory to it, make_stuff_document_runnable
• Basic unit tests for this
• A notebook to work on summarisation functions going forward

Guidance to review

• Read the unit tests and check you're happy with what they do
• Check the runnable function signature and check you're happy with how that works
• Watch the unit tests pass, approve

Things to check

• I have added any new ENV vars in all deployed environments
• I have tested any code added or changed
• I have run integration tests -- ⚠️ End to end is fine, journey isn't -- suspect this branched from a pre-fix

[wpfl-dbt]

Taking back to draft -- overcomplicated merging. Can do with Chunk, not LangChain objects

[wpfl-dbt]

Back to proper PR!

[andy-symonds]

Approving, and we keep an eye on the integration tests - journey should be fixed from changes already merged to main.