perf(core): optimize broken links checker

[slorber]

Motivation

Fix #9754

The new logic added for the broken anchor link checker made the algorithm slow for large sites with many routes and links.

This PR optimizes the algo:

• reduces the number of useless matchRoutes(routes,pathname) calls
• reduces the number of routes to match against when possible
• uses more scalable Set/Map data structures

Test Plan

CI + unit tests

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Considering this site/commit: harness/developer-hub@d5d1311

Upgrading to Docusaurus v3.1 leads the broken link checker to take 5 minutes. After this PR it only takes 1 second.

Another site reporting significant build time improvements:
#9754 (comment)

[netlify]

✅ [V2]

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 74	🟢 98	🟢 100	🟢 100	🟠 89	Report
/docs/installation	🟠 89	🟢 98	🟢 100	🟢 100	🟠 89	Report
/docs/category/getting-started	🟠 76	🟢 100	🟢 100	🟢 90	🟠 89	Report
/blog	🟠 71	🟢 100	🟢 100	🟢 90	🟠 89	Report
/blog/preparing-your-site-for-docusaurus-v3	🟠 64	🟢 97	🟢 100	🟢 100	🟠 89	Report
/blog/tags/release	🟠 71	🟢 100	🟢 100	🟠 80	🟠 89	Report
/blog/tags	🟠 76	🟢 100	🟢 100	🟢 90	🟠 89	Report

[github-actions]

Size Change: 0 B

Total Size: 986 kB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	75.1 kB
website/build/assets/css/styles.********.css	114 kB
website/build/assets/js/main.********.js	759 kB
website/build/index.html	37.9 kB

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