[docs] Redesign

[stevhliu]

The main goal of this PR is to redesign the Transformers docs to:

1. Be more developer-friendly.
2. Improve navigation by replacing the existing structure with a more organic one that scales naturally instead of forcing content into the 4 current predefined sections.
3. Create a more unified docs experience by integrating content rather than adding it on.

This PR proposes a potential structure for achieving 2 and 3. Once the structure is in place, each doc will be rewritten to achieve 1.

If you're interested in more details about the redesign's motivation, please read this blog post. If you want more details about 1, 2, and 3, please read this post and this one too.

All feedback, alternative structures, and comments are welcomed! Thanks 🙂