Implement link validator and update gitignore for caching

[Amaru333]

This pull request introduces automated link checking to the CI workflow, improves link validation tooling, and adds scripts for easier local and CI-based link checks across all app workspaces. The changes ensure that both internal and external links are validated regularly, and provide developers with convenient scripts to run these checks locally.

CI/CD and Workflow Enhancements:

• Added scheduled and manual link checking jobs to .github/workflows/ci.yaml, including detection of changed apps, internal link checks for all apps, and external link checks for site and consortium. This ensures broken links are caught both on PRs and on a weekly schedule. [1] [2]
• Updated the lint job to only run on relevant events, optimizing workflow runs.

Developer Tooling and Scripts:

• Added a new script scripts/check-links.mjs that builds each app and runs the Lychee link checker in offline, online, or unique-link modes, making it easy to validate links locally or in CI.
• Added links:offline, links:online, and links:unique npm scripts to the root package.json and to each app (site, consortium, docs) for running link checks directly via npm. [1] [2] [3] [4]

Configuration and Documentation:

• Introduced a project-wide Lychee configuration file (lychee.toml) to standardize link checking behavior and exclusions.
• Updated README.md to document the new link check in the CI process, clarifying when internal and external links are validated.

[mbwatson]

a nice improvement overall, especially keeping link checks non-blocking so they report issues and don't block releases

we may modify on.push to push: { branches: ['**'] } to not fire on tag pushes, which is likely in our future.

could we add a short note in README.md about local Lychee installation prerequisites for the new links:* scripts? Right now scripts/check-links.mjs invokes Lychee directly, so devs may hit a "command not found" error. a brief "install Lychee + verify with lychee --version" note for Mac and Linux could help smooth things out.