chore: Added symlink to AGENTS.md for copilot (#16663)

## DESCRIBE YOUR PR

Adding a symlink to the AGENTS.md file that will be picked up by the
copilot extension when using this to work with LLMs in vscode etc.

## IS YOUR CHANGE URGENT?  

- None: Not urgent, can wait up to 1 week+

## SLA

cc: @Flash0ver 

## 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)

Author: jamescrosswell

.github/copilot-instructions.md

@@ -0,0 +1 @@
+../AGENTS.md