docs: Add comprehensive Nix cache documentation

[joereg4]

Fixes #1622

Description

Adds comprehensive documentation for the Nix binary cache system used in the Supabase PostgreSQL project.

Changes

• Created nix/docs/nix-cache.md with comprehensive cache documentation
• Updated nix/docs/README.md to include cache docs in navigation
• Updated nix/mkdocs.yml to add cache docs to MkDocs navigation
• All tests pass (nix flake check -L)

Closes #1622"

[joereg4]

I noticed the CI failed due to missing AWS credentials. Are contributors supposed to receive credentials or should those steps skip in forks / PRs?

[joereg4]

Hello @samrose,

I'm a new contributor working on a PR for issue #1622 (Nix cache documentation). I've encountered the AWS credentials CI failure on my fork's PR and wanted to reach out for some guidance.

I've observed that the run-testinfra jobs fails with DEV_AWS_ROLE credential errors on fork PRs. This appears to be GitHub's security model preventing forks from accessing repository secrets. Looking through PRs and I do see this is impacting others as well.

All my local and other CI tests pass and the code changes are documentation-focused (new nix-cache.md file).

Since you are the #1 contributor, what's the best way forward for contributors facing this CI limitation? Should I wait for a maintainer to manually review/approve the AWS tests, or is there something else I should be doing?