(core) Document configuring AI assistance

Summary:
Added OPENAI_API_KEY and GRIST_FORMULA_ASSISTANT to the README. GRIST_FORMULA_ASSISTANT may be removed in the long term, but for now the goal is just to get something into the grist-core README quickly for self-hosters.

Removed `documentation/llm.md` because it's outdated and not really providing value.

Test Plan: none

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3963

README.md

@@ -296,6 +296,7 @@ PORT                | port number to listen on for Grist server
 REDIS_URL           | optional redis server for browser sessions and db query caching
 GRIST_SNAPSHOT_TIME_CAP       | optional. Define the caps for tracking buckets. Usage: {"hour": 25, "day": 32, "isoWeek": 12, "month": 96, "year": 1000}
 GRIST_SNAPSHOT_KEEP           | optional. Number of recent snapshots to retain unconditionally for a document, regardless of when they were made
+OPENAI_API_KEY      | optional. Used for the AI formula assistant. Sign up for an account on OpenAI and then generate a secret key [here](https://platform.openai.com/account/api-keys). You also need to set `GRIST_FORMULA_ASSISTANT=1`.
 
 Sandbox related variables:
 

documentation/llm.md

@@ -1,35 +0,0 @@
-# Using Large Language Models with Grist
-
-In this experimental Grist feature, originally developed by Alex Hall,
-you can hook up OpenAI's ChatGPT to write formulas for
-you. Here's how.
-
-First, you need an API key. Visit https://openai.com/api/ and prepare a key, then
-store it in an environment variable `OPENAI_API_KEY`.
-
-That's all the configuration needed!
-
-Currently it is only a backend feature, we are still working on the UI for it.
-
-## Hugging Face and other OpenAI models (deactivated)
-
-_Not currently available, needs some work to revive. These notes are only preserved as a reminder to ourselves of how this worked._
-
-~~To use a different OpenAI model such as `code-davinci-002` or `text-davinci-003`,
-set the environment variable `COMPLETION_MODEL` to the name of the model.~~
-
-~~Alternatively, there are many non-proprietary models hosted on Hugging Face.
-At the time of writing, none can compare with OpenAI for use with Grist.
-Things can change quickly in the world of AI though. So instead of OpenAI,
-you can visit https://huggingface.co/ and prepare a key, then
-store it in an environment variable `HUGGINGFACE_API_KEY`.~~
-
-~~The model used will default to `NovelAI/genji-python-6B` for
-Hugging Face. There's no particularly great model for this application,
-but you can try other models by setting an environment variable
-`COMPLETION_MODEL` to `codeparrot/codeparrot` or
-`NinedayWang/PolyCoder-2.7B` or similar.~~
-
-~~If you are hosting a model yourself, host it as Hugging Face does,
-and use `COMPLETION_URL` rather than `COMPLETION_MODEL` to
-point to the model on your own server rather than Hugging Face.~~