chore: Remove own build pipeline from docs

[baseballyama]

related: #824

The goals of this PR are:

• Remove all proprietary build processes, including markdown processors
  • Use normal markdown as much as possible. The use of Svelte components in markdown files is no longer possible.
• Remove ESLint from the browser
• Make the docs project an ESM

This will make migration to omnisite much easier.

[changeset-bot]

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Latest commit: 2abcee4

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