docs: reference plugin-tools llms.txt from grafana.com/llms.txt

[tolzhabayev]

Summary

We already publish a plugin-tools-specific llms.txt at https://grafana.com/developers/plugin-tools/llms.txt, but the top-level https://grafana.com/llms.txt does not reference it. As a result, LLM agents that discover Grafana via the standard /llms.txt entry point have no path to plugin development docs.

Context

llms.txt is becoming a de facto standard for guiding LLM agents to canonical documentation. Chrome / Lighthouse now recommends it as part of agentic browsing: https://developer.chrome.com/docs/lighthouse/agentic-browsing/llms-txt

The current grafana.com/llms.txt indexes core stack, observability, IRM, connect/visualize, etc. — but has no "Developers" or "Plugins" section, and no link to the plugin-tools sub-index that already exists.

Proposal

Add a ## Developers (or ## Plugin development) section to grafana.com/llms.txt that links to the plugin-tools llms.txt, e.g.:

## Plugin development
- [Plugin tools](https://grafana.com/developers/plugin-tools/llms.txt): Grafana plugin development — create-plugin, e2e testing, how-to guides, migration, publishing.

The grafana.com/llms.txt is generated/maintained outside this repo, so this issue is mainly to track the ask from the plugin-tools side and hand off to whoever owns the top-level file (likely website / docs platform).