docs: provide llms.txt

[JosiahParry]

{pkgdown} now can now create an llms.txt (see rlang's llms.txt).

It would be great if one was provided for {cli}.

I can never quite remember the classes and rules. It would be nice to be able to feed this to claude to help me improve my messaging.

[JohnnyLagneau]

Supportive of this. For docs/API-style GEO, the highest-signal version is usually not a giant dump of files but a routing layer for real developer jobs.

A strong setup here would be:

• short llms.txt with what {cli} is for, core primitives, and the most-cited docs entry points
• optional llms-full.txt for broader coverage
• a few answer-first pages for high-intent tasks (alerts, progress bars, inline styles, pluralization, theming)
• stable headings/titles so retrieval systems can cite the right page cleanly
• explicit “when to use X vs Y” guidance, since that’s where LLM answers often drift

That tends to improve citation quality and implementation accuracy more than exporting every markdown file raw.