fix(docs, blog): Markdown link resolution does not support hot reload

[slorber]

Motivation

Fix #10152

The bug only happens during the dev experience because we "hot reload" the site.

When we create 1.md and then reference it with a link such as [1][./1.md] is another file, we get a markdown link resolution error.

This happens because the "sourceToPermalink" map (that we use for resolving markdown links) is not updated when we reload/re-read the site's files, for both the docs and blog plugins.

Note: the solution I applied is not elegant, and relies on mutable map update side-effects. I don't know yet how to solve it properly considering the configureWebpack hook is only called once and it's not really safe to read loadedContent from there, but I'll try to figure this out! Maybe this will require creating a new plugin lifecycle or using some moreadvanced Webpack features? 🤷‍♂️

Note: I also refactored the data structure from Record<string,string> to Map<string,string> for perf reasons.

Test Plan

Unit tests keep passing + my local experience has

Test links

https://deploy-preview-10185--docusaurus-2.netlify.app/

[netlify]

✅ [V2]

Name	Link
🔨 Latest commit	112ff4f
🔍 Latest deploy log	https://app.netlify.com/sites/docusaurus-2/deploys/6659e95f5c43e600081e9da7
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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 68	🟢 98	🟢 96	🟢 100	🟠 88	Report
/docs/installation	🟠 59	🟢 96	🟢 100	🟢 100	🟠 88	Report
/docs/category/getting-started	🟠 75	🟢 100	🟢 100	🟢 90	🟠 88	Report
/blog	🟠 69	🟢 100	🟢 100	🟢 90	🟠 88	Report
/blog/preparing-your-site-for-docusaurus-v3	🟠 62	🟢 96	🟢 100	🟢 100	🟠 88	Report
/blog/tags/release	🟠 69	🟢 100	🟢 100	🟢 90	🟠 88	Report
/blog/tags	🟠 76	🟢 100	🟢 100	🟢 90	🟠 88	Report

[netlify]

✅ [V2]

Name	Link
🔨 Latest commit	39caf6b
🔍 Latest deploy log	https://app.netlify.com/sites/docusaurus-2/deploys/6659ef86d6613a0008804e66
😎 Deploy Preview	https://deploy-preview-10185--docusaurus-2.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

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[github-actions]

Size Change: 0 B

Total Size: 1.72 MB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/codeTranslations.json	2 B
website/.docusaurus/docusaurus.config.mjs	27.1 kB
website/.docusaurus/globalData.json	107 kB
website/.docusaurus/i18n.json	930 B
website/.docusaurus/registry.js	276 kB
website/.docusaurus/routes.js	180 kB
website/.docusaurus/routesChunkNames.json	120 kB
website/.docusaurus/site-metadata.json	2.17 kB
website/build/assets/css/styles.********.css	112 kB
website/build/assets/js/main.********.js	855 kB
website/build/index.html	38.1 kB

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