feat: create llms.txt generator

[araujogui]

Description

Creates a llms.txt generator

Validation

npx api-docs-tooling generate  -i doc/api/*.md -t llms-txt -o out/

Related Issues

Fixes #226

Check List

• I have read the Contributing Guidelines and made commit messages that follow the guideline.
• I've covered new added functionality with unit tests if necessary.

[AugustinMauroy]

Could you add some test this is not a legacy thing.

[AugustinMauroy]

LGMT ! that's cool

[araujogui]

I'm not sure using the first paragraph node as the description is a good idea. Here's the actual llms.txt—some of the descriptions seem incorrect: https://pastebin.com/edEGgFLy

Maybe we could add another yaml property like llm-description to api docs

[ovflowd]

Minor nits but LGTM!

[ovflowd]

Final review pass! (There's 2 open threads, feel free to resolve them when you're ready and merge the PR)