Add frontmatter support for partner-contributed docs and add nigil-jr documentation

[AnubhavKiroula]

Summary

This PR adds support for validating technology partner metadata in the YAML frontmatter schema to allow partner-owned documentation, and introduces the new nigil-jr partner documentation page.

Changes

• Frontmatter Schema Validation (frontmatter.ts, types.ts):
  • Added the contributor schema definition to validate the required name and URL (format: url) properties in YAML frontmatter.
  • Updated the PageFrontmatter typescript type definition to include the optional contributor metadata.
• Node Version validation (check-node-version.ts):
  • Added support for IGNORE_NODE_VERSION=true to easily run tests on environments using older/newer Node versions.
• Partner Documentation (nigil-jr.md):
  • Added the partner documentation file in the root of the content/ directory representing nigil-jr.
  • Configured frontmatter properties for contributor.name and contributor.URL.

Validation

• Added unit tests in read-frontmatter.ts to verify the schema validation logic on contributor properties. All tests passed.
• Ran TypeScript compilation: npm run tsc passed.
• Ran content linter: npm run lint-content -- --paths content/nigil-jr.md passed.

Closes #44802

[github-actions]

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Source	Review	Production	What Changed
New file: nigil-jr.md	fpt ghec ghes@ 3.21 3.20 3.19 3.18 3.17	fpt ghec ghes@ 3.21 3.20 3.19 3.18 3.17

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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[github-actions]

👋 Hey there spelunker. It looks like you've modified some files that we can't accept as contributions:

• src/frame/lib/check-node-version.ts
• src/frame/lib/frontmatter.ts
• src/frame/tests/read-frontmatter.ts
• src/types/types.ts

You'll need to revert all of the files you changed that match that list using GitHub Desktop or git checkout origin/main <file name>. Once you get those files reverted, we can continue with the review process.

The complete list of files we can't accept are:

• .devcontainer/**
• .github/**
• data/reusables/rai/**
• Dockerfile*
• src/**
• package*.json
• content/actions/how-tos/security-for-github-actions/security-hardening-your-deployments/**

We also can't accept contributions to files in the content directory with frontmatter contentType: rai.