feat(v2): generalize usage of _ prefix convention to exclude content files/folders

[slorber]

Breaking change

• we now use the _ prefix as a default convention to prevent content from creating pages
• if you already have files/folders starting with _ it might cause troubles in your system. You can revert to previous behavior by using the new exclude config (onall of blog/docs/pages) to remove that _ prefix convention
• https://docusaurus.io/docs/next/api/plugins/@docusaurus/plugin-content-docs#configuration

        exclude: [
-         '**/_*.{js,jsx,ts,tsx,md,mdx}',
-         '**/_*/**',
          '**/*.test.{js,jsx,ts,tsx}',
          '**/__tests__/**',
        ],

Motivation

Generalize the usage of a _ file/folder prefix to ignore the creation of routes/pages for a given file, in all 3 content plugins.

This is mostly useful to co-locate partials and reusable components near the page/doc using them.

The following files should not lead to any created route:

Pages:

• src/pages/_partial.md
• src/pages/_partials/partial.md
• src/pages/_partials/subfolder/partial.md
• src/pages/_Component.js
• src/pages/_Components/Components.js
• src/pages/_Components/subfolder/Component.js

Docs:

• docs/_partial.md
• docs/_partials/partial.md
• docs/_partials/subfolder/partial.md

Blog:

• blog/_partial.md
• blog/_partials/partial.md
• blog/_partials/subfolder/partial.md

This feature was previously only partially supported in the page plugin (not supporting _folder). Now it's consistent everywhere.

Have you read the Contributing Guidelines on pull requests?

yes

Test Plan

tests

dogfood:
http://localhost:3000/docs/_partials/swizzleWarning
http://localhost:3000/docs/cli#docusaurus-swizzle-sitedir
http://localhost:3000/docs/using-themes#swizzling-theme-components

[netlify]

✔️ [V2]

🔨 Explore the source changes: f9222e2

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/60f0182a2578b7000883a9c2

😎 Browse the preview: https://deploy-preview-5173--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟢 Performance	98
🟢 Accessibility	98
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-5173--docusaurus-2.netlify.app/

[github-actions]

Size Change: 0 B

Total Size: 850 kB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	58.8 kB
website/build/assets/css/styles.********.css	89 kB
website/build/assets/js/main.********.js	459 kB
website/build/blog/2017/12/14/introducing-docusaurus/index.html	62.4 kB
website/build/blog/index.html	26.3 kB
website/build/docs-tests/index.html	20.3 kB
website/build/docs-tests/standalone/index.html	18.8 kB
website/build/docs/index.html	40.2 kB
website/build/docs/installation/index.html	47.9 kB
website/build/index.html	27.3 kB

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