feat(LLMs): Added llms.txt and SKILL.md for better LLMs friendliness (#17762)

## DESCRIBE YOUR PR
- Added new script (generate-llms-txt.mjs) to generate an llms.txt file
that will be available publicly on the docs site.
- Created an new SKILL.md file that references our sentry-for-ai repo
and our skills.sentry.dev site, and points to our AGENTS.md and other
skill pages within the repo.

Preview:
https://sentry-docs-git-feat-llms-txt-and-root-skill.sentry.dev/llms.txt

**Why?**
This will help LLMs have better guidance navigating our docs, as well as
finding the appropriate skills and references for configuration. It will
also help LLMs get the proper instructions on how to stay up to date
with the latest about how Sentry works, including new features. It also
directly points to https://skills.sentry.dev and the `sentry-for-ai`
repo.

## IS YOUR CHANGE URGENT?  

Help us prioritize incoming PRs by letting us know when the change needs
to go live.
- [ ] Urgent deadline (GA date, etc.): <!-- ENTER DATE HERE -->
- [ ] Other deadline: <!-- ENTER DATE HERE -->
- [x] None: Not urgent, can wait up to 1 week+

## SLA

- Teamwork makes the dream work, so please add a reviewer to your PRs.
- Please give the docs team up to 1 week to review your PR unless you've
added an urgent due date to it.
Thanks in advance for your help!

## PRE-MERGE CHECKLIST

*Make sure you've checked the following before merging your changes:*

- [ ] Checked Vercel preview for correctness, including links
- [ ] PR was reviewed and approved by any necessary SMEs (subject matter
experts)
- [ ] PR was reviewed and approved by a member of the [Sentry docs
team](https://github.com/orgs/getsentry/teams/docs)

## LEGAL BOILERPLATE

<!-- Sentry employees and contractors can delete or ignore this section.
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Look, I get it. The entity doing business as "Sentry" was incorporated
in the State of Delaware in 2015 as Functional Software, Inc. and is
gonna need some rights from me in order to utilize my contributions in
this here PR. So here's the deal: I retain all rights, title and
interest in and to my contributions, and by keeping this boilerplate
intact I confirm that Sentry can use, modify, copy, and redistribute my
contributions, under Sentry's choice of terms.

## EXTRA RESOURCES

- [Sentry Docs contributor guide](https://docs.sentry.io/contributing/)

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Co-authored-by: Shannon Anahata <shannonanahata@gmail.com>
Co-authored-by: getsantry[bot] <66042841+getsantry[bot]@users.noreply.github.com>