Improve documentation

[HugoGranstrom]

Currently the documentation consists of the README and the example slides in docs/. This could be improved, both by having a compiler-generated API docs and more specific example slides in the style of NimConf, where a piece of code is shown next to what it renders. Ideas for what these small tutorials should cover are welcomed.

This was brought up by @srozb .

• Compiler-generated docs
  • Add doc-comments to all important procs/templates.
  • Add a short introduction at the top.
• Landing page for documentation that links to the docs and all available tutorials.
  • Rename index.nim to something else. Or should I just remove it? It is quite messy and showcase.nim already covers everything it does.
  • Move content from README to here instead to de-clutter the README.
• Make a nimiBook for the tutorials instead! Then it will be easier to find stuff!
• Embedded slideshows like Reveal.js has in their docs

Ideas thus far:

• Basics (nbText, nbCode, nbImage, columns)

[HugoGranstrom]

Write about gotchas:

• If too many slides is create a [GC] cannot register global variable; too many global variables error will be obtained. The solution is to split the slides into multiple slideshows or help make nimib use less global variables.
• Exporting to PDF requires Chromium and the export has a few odd behaviors like embedding a file multiple times. So cornerImages and background images can explode the exported file size.