Clarify local validation commands in CONTRIBUTING.md

[miqui]

List the individual npm scripts (validate-markdown, format-markdown, build-src, test) so contributors can pick the fast lint/link-check loop while editing versus the full build/test pass before opening a PR.

• no schema changes are needed for this pull request

[handrews]

@miqui please make sure this won't conflict with #5424 which updates this documentation based on changes to the scripts due to shared build infrastructure. It might be easier to update that part after getting the build system updated.

[miqui]

@handrews good call, will check, thanks

[miqui]

hi @handrews - just checked your PR and there is no conflict. In package.json it keeps the exact same npm script names I documented in my PR: validate-markdown, format-markdown, build-src, test. Your PR is only swapping their internal implementation from inline npx/bash calls to thin wrappers around a new @oai/build-infra CLI (oai-spec-validate-markdown, etc.).

We are good.