Harmonize documentation (and move it to a more familiar grounds)

[CsatariGergely]

We have documentation in different places:

• https://kpt.dev/
• https://catalog.kpt.dev/
• https://kptdev.github.io/kpt/reference/ - non active

The active ones are using a js based framework what is unfamiliar for me.

I would like to collapse the catalog documentation to the main kpt in a docs repo and use the Hugo, Docsify framework to render.
What do we think?