feat(): add relative link validation and update cross-topic references

[dobromirts]

Closes #330, #331

1. Added check-relative-links:ci npm script and updated the lint CI workflow to validate all relative cross-topic links at build time -generates angular and xplat content first, then checks both trees so no links to generated pages are missed
2. Extended the checker to accept bare page.mdx links alongside ./page.mdx, excluded _shared/ template directories from direct scanning (links validated via generated output), and added docs/xplat/generated/ to the xplat scan path
3. Updated README, CONTRIBUTING, and PR template to document the link convention and pipeline

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[ChronosSF]

@dobromirts , you'll have to resolve some conflicts while this is being tested ;)

PR looks good to me on the basis of a quick go-through the files. I'll test how the checker works now.

[dobromirts]

latest reports visible here - reports/relative-links-report.md and reports/html-links-report.md