Add robots.txt for search engine indexing

[steveatlc]

Summary

• Adds robots.txt with sitemap reference to help search engines discover and index the documentation site

Test plan

• Verify https://docs.limacharlie.io/robots.txt returns correctly after deploy

🤖 Generated with Claude Code

[github-actions]

Broken Links Detected

The link checker found some issues:

* [ERROR] <file:///home/runner/work/documentation/documentation/site/1-getting-started/what-is-limacharlie/use-cases> | Cannot find file: File not found. Check if file exists and path is correct
* [ERROR] <file:///home/runner/work/documentation/documentation/site/3-detection-response/tutorials/3-detection-response/sensor-variables.md> | Cannot find file: File not found. Check if file exists and path is correct
* [ERROR] <file:///home/runner/work/documentation/documentation/site/3-detection-response/tutorials/4-data-queries/template-transforms.md> | Cannot find file: File not found. Check if file exists and path is correct
* [ERROR] <file:///home/runner/work/documentation/documentation/site/3-detection-response/tutorials/8-reference/detection-logic-operators.md> | Cannot find file: File not found. Check if file exists and path is correct
* [ERROR] <file:///home/runner/work/documentation/documentation/site/3-detection-response/tutorials/8-reference/endpoint-commands.md> | Cannot find file: File not found. Check if file exists and path is correct
* [ERROR] <file:///home/runner/work/documentation/documentation/site/3-detection-response/tutorials/9-ai-sessions/dr-sessions.md> | Cannot find file: File not found. Check if file exists and path is correct
* [ERROR] <file:///home/runner/work/documentation/documentation/site/5-integrations/outputs/stream-structures/destinations> | Cannot find file: File not found. Check if file exists and path is correct
* [ERROR] <file:///home/runner/work/documentation/documentation/site/6-developer-guide/7-administration/access/secrets.md> | Cannot find file: File not found. Check if file exists and path is correct

Please fix these broken links before merging.

Full results

See the uploaded artifact for complete link check results.