feat: add sitemap.xml and llms.txt (#138)

## Summary by Sourcery

Add a sitemap and an llms.txt file for improved SEO and LLM indexing.

New Features:
- Added a sitemap.xml file to improve SEO.

Tests:
- Added a plugin to generate an llms.txt file containing all
documentation and blog post links, which is helpful for LLM indexing.

Author: roderik