Update documentation tools

Here is a summary of how it works

 * The templating now fully uses Razor and there is just a single template. In fact, the template is the same as the one in F# formatting ([here](https://github.com/tpetricek/FSharp.Formatting/tree/master/misc/templates)). 

 * The F# Formatting package contains more templates that can be overridden by copying them to the local `templates` folder (but these are mainly used for API reference and that is still changing, so it is good idea to use default templates in F# Formatting for that). 

 * The template that is included in the scaffold is the main one and defines the overall page look. It has some `@Properties[...]` tags to get information from the parameters passed from `generate.fsx` - so by default, you'll get a decent web page even when you do not touch the template.

 * The `generate.fsx` script can be used in F# interactive (select all - Alt+Enter), in which case it generates documentation with local links (there is an `#if` for that). When called from `build.fsx`, it uses the live URL, so the generated docs can be pushed to Github.

 * The script also copies default styles and JavaScript for tooltips from the files included in the F# Formatting package (to change these, you'd have to remove line 63-64, but it is probably better to add custom CSS and override this).

Aside from that, I did some minor changes in `build.fsx` (mainly to make it a bit simpler - or at least to make it look simpler when you look at the first few lines). But feel free to undo anything there (except for the `--define` parameter to FSI).