chore: add CI tests workflow

[araujof]

Summary

Adds CI test infrastructure and documentation build checks.

CI now runs tests and validates docs on PRs, while the deploy workflow was renamed for clarity.

Closes: #36

Changes

• New tests.yaml workflow — runs Go tests with race detection across go/ subdirectories, with caching.
• New docs-build.yaml workflow — validates the Hugo docs site builds cleanly on PRs (without deploying).
• Updated ci.yml — added a step (likely linting or checks).
• Renamed docs.yaml → docs-deploy.yaml — clarifies that the existing workflow is for deployment, distinct from the new build-check workflow.
• Minor docs tweak in _index.md.

[terylt]

LGTM