Document how to add custom model providers

[krassowski]

References

Relates to issues:

• Custom local LLMs #389
• Update prompts for locally installed models #226

Code changes

• when loading model via entry points fails, display the error message in the logs
• fix a typo in a debug log

User-facing changes

Adds a documentation section describing a minimal, two-file custom LLM provider, with links to upstream (LangChain) documentation for further reading.

Demo of the model

[JasonWeill]

Thank you so much for improving the docs! A few small changes, otherwise looking good, thanks.

[dlqqq]

Awesome, thank you @krassowski!

[JasonWeill]

@meeseeksdev please backport to 1.x

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

git checkout 1.x
git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

git cherry-pick -x -m1 7e4a2a5115ea691afb68522ec267d879e8b3fc95

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

git commit -am 'Backport PR #420: Document how to add custom model providers'

4. Push to a named branch:

git push YOURFORK 1.x:auto-backport-of-pr-420-on-1.x

5. Create a PR against branch 1.x, I would have named this PR:

"Backport PR #420 on branch 1.x (Document how to add custom model providers)"

And apply the correct labels and milestones.

Congratulations — you did some good work! Hopefully your backport PR will be tested by the continuous integration and merged soon!

Remember to remove the Still Needs Manual Backport label once the PR gets merged.

If these instructions are inaccurate, feel free to suggest an improvement.