Improve default styling and top-level content layout

As noted by @baronfel in #27, the default styling and top-level layout needs improvement.

What is here is the current default styling for FSharp.Formatting using the standard _template.html, which is tight for iteration but we can do better. If necessary we can add a `_template.html` or `fsdocs-styles.css` in here though to be honest I'd love it if we could make the default FSharp.Formatting _template.html good enough to be both configurable for the needs of this repo and to satisfy and please.

Please make a note below everything that you do/don't like about the current design, layout and content. If possible give specific (preferably visual) examples of what you want improved, and, if possible, give links to how those improvements might be made in terms of CSS improvements and JS framework changes.

The things I've noticed so far:

- The scrollbar on the left appears even if its not needed. This is I believe because of horizontal overflow.

- Agreed with @baronfel the basic look is very sparse (though I quite enjoyed browsing it on my phone earlier). Also this is tricky as the site is not like most other doc sites as it is *only* doing FSHarp.Core API docs and is not a general tech doc site.

- The repetition of parameter XML doc with parameter types is a bit painful (this is an F#F issue)

- There is a glitch iwhere function names and parameters run together for active patterns and some module functions

- Agreed with @cartermp that the link on the logo should go to https://fsharp.org