docs: add Sourcey-generated llms.txt

[codexbounty260804]

Closes #29.

This adds a generated llms.txt at the MkDocs site root so coding agents can discover PgDog's installation, administration, pooling, load-balancing, and sharding documentation without crawling the full site.

Generation details:

• Source: this repository at d598e06d619fc90893e06ced1cce70956aaca441
• Generator: Sourcey 3.6.5
• Output: 62 documentation entries selected from the existing Markdown source
• Validation: all 62 generated targets returned HTTP 200 after redirects; the output was reproduced in a governed runx 0.8.2 build with matching SHA-256 c3fcdfcf39add8342b9aecc2b9101c065e5847c009330f7a176bdf92975a803b

The file is static, so it deploys through the repository's existing MkDocs workflow without adding a runtime dependency.

[levkk]

Oh cool, thanks! Would be cool to include some kind of script so we can generate it when we change the docs, but this is a good start.

[codexbounty260804]

Added this in the latest commits:

• scripts/generate_llms.py pins Sourcey 3.6.5 and updates only docs/llms.txt.
• docs/sourcey.config.ts records the curated 62-page source set.
• The README now documents the regeneration command.

I ran the script twice: the second run was idempotent, and the output remains SHA-256 c3fcdfcf39add8342b9aecc2b9101c065e5847c009330f7a176bdf92975a803b with 62 links.

[codexbounty260804]

@levkk The requested regeneration script and pinned Sourcey config are in, the output is idempotent, and all 62 generated links were rechecked. This is ready for review; I can turn around any follow-up quickly.