refactor(v2): use absolute path instead of alias for metadata source

[endiliey]

Motivation

Current metadata for docs in v2 form is like below:

const metadata = {
    id: 'en-permalink',
    title: 'Permalink',
    permalink: '/docs/en/endiliey/permalink',
    language: 'en',
    source:
      '@docs/permalink.md',
    localized_id: 'permalink',
}

This is because I use @docs alias in webpack, but I guess this could be confusing and extra aliasing work. Let's just use absolute import so the generated sstructure is like below:

const metadata = {
    id: 'en-permalink',
    title: 'Permalink',
    permalink: '/docs/en/endiliey/permalink',
    language: 'en',
    source:
      '/mnt/c/Users/endij/Desktop/Linux/Docusaurus/v2/test/__fixtures__/docs/permalink.md',
    localized_id: 'permalink',
  }

Extra: added stronger test for docs processing in v2

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

cd v2
yarn
yarn start

All the docs still working including relative linking

[docusaurus-bot]

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