chore: Remove own build pipeline from docs

[baseballyama]

related: https://github.com/sveltejs/eslint-plugin-svelte/issues/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[bot]]

⚠️ No Changeset found

Latest commit: 2abcee4731a6bd9e1873d23f72e66ad59c61f820

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR