Improving Kopia's documentation/website

Hi

As many of you have noticed the documentation is not really in great shape and as you’re going through onboarding experience there are many rough edges.

I would greatly appreciate if you folks could help improve the site by submitting PRs with fixes for the site every time you encounter an issue. This is quite simple - the site is written in Markdown.

If you’re a technical writer (or know somebody who is) and are willing to some time on reviewing, improving and organizing the docs (either one-time or on a regular basis), that would also be great help.

Build and testing instructions are in https://github.com/kopia/kopia/blob/master/BUILD.md

Not a technical writer, but will give it a shot. Specially on areas I am seeing missing such as the SFTP section.