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.
• [x] Landing page for documentation that links to the docs and all available tutorials.
  • [x] Rename index.nim to something else. Or should I just remove it? It is quite messy and showcase.nim already covers everything it does.
  • [x] Move content from README to here instead to de-clutter the README.
• [x] Make a nimiBook for the tutorials instead! Then it will be easier to find stuff!
• [x] Embedded slideshows like Reveal.js has in their docs

Ideas thus far:

• [x] Basics (nbText, nbCode, nbImage, columns)

[HugoGranstrom]

Write about gotchas:

• [x] 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.
• [x] 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.