Create "Nimble Guide"

[narimiran]

This is a large PR, so here is a TL;DR — instead of having the documentation in the (very large) readme, it is separated into a proper multi-page documentation, with a working search, etc etc.

You can see how it looks here: https://narimiran.github.io/nimble/ https://nim-lang.github.io/nimble/

The documentation is, roughly, divided into three sections (see the menu at the top of the index page):

• using existing packages
• creating a new package
• how to use the new nimble develop workflow

IMO, the content itself can (and should) be improved once this is merged, so others can contribute.

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UPDATE: Ready for merging.

This is NOT READY for merging. The following steps are needed before it should be merged:

1. The admin should go to the repo settings and under 'pages' tab enable gh-pages, see the pic below on how it should look: Done.
   

2. Once this is done (please ping me), I need to make another commit, changing the remote repo from my fork to this repo in the Makefile and changing the links in readme. Done.