[FEATURE]: Revamp docs and make them more visually appealing

[renardeinside]

As UCX project grows, a single README.md becomes cluttered and makes a lot of things unclear to the end users.
A proper documentation with appealing visuals would help users to better understand the UCX and apply it.

[nfx]

there's a plan to improve docs, but it's not the highest on the list now. let's use github votes on this issue as a measure of the real customer need - if it reaches

it is wider than just this repository and documentation infrastructure will work with all projects in /databrickslabs org, not just UCX, so docusaurus is not the one high on the list.

we also need docs in the readme as part of the PyPI release, though they still require some post-processing to ToC to render properly, but it'll be done in the same push.