Introduce dedicated documentation

[renardeinside]

Introduces a full-scale documentation based on:

• Hugo
• Hextra
• TailwindCSS

Docs structure is split into 4 sections:

User-facing:

• Installation
• Process (describes actual migration process)
• Reference (describes commands and details)

Also, there is a separate dev docs section.

Some screenshots:

Landing page:

Overview:

Installation instructions:

Search is built-in, as well as many other capabilities.

There are some unresolved problems however:

1. Hugo doesn't have any link verification mechanism at all for fragment-based links (e.g. #some-fragment).
2. We require a lot of custom code to simply render markdown-like links.
3. Hextra theme doesn't have "zoom-on-click" functionality. We'll probably need to implement it.

[renardeinside]

closing this one in favor of #3458 due to framework change.