Consolidate documentation

[ningvin]

The second part of my series of documentation improvements discussed here: #292 / #285.

This PR addresses the following:

• Converted the tutorial from HTML to Markdown
• Converted most stuff from the website to Markdown
• Converted @carenas' overview to Markdown
• Reworked the overall structure
• Added some general / setup specific documentation

[zherczeg]

This looks like a large work, thank you. At some point please squash the commits into one.

[zherczeg]

Can you add html elements to markdown? Is it a standard?

[ningvin]

Well, sort of. GitHub does (see their GitHub Flavored Markdown spec) and so do pretty much all Markdown flavors that are based on the CommonMark spec (which is most of them).

The usage of HTML here is just for eye candy though, so we could run with plain Markdown just as well

[zherczeg]

This looks like the repeat of readme. When somebody modify one of them, it is easy to forget about the other. Would be good to figure out something for this.

[ningvin]

Not sure if there is a nice solution for this. I could remove the info from the introduction, but I would definitely keep it in the README as it is the primary entry point for people looking at this project.