docs: link Prolly repository references

[forhappy]

Summary

• Add direct links to the Prolly repository at the first relevant mention in the README, API guide, client and core guides, architecture references, changelog, and contribution guide.
• Clarify the boundary between SILO's S3-backed repository workflows and the Prolly tree model.
• Keep the existing terminology and compatibility notes intact.

Validation

• git diff --check HEAD^ HEAD
• Audited all eight changed documents for the Prolly repository URL.

This is a documentation-only change and is opened as a draft for review.