Documentation

# Introduction
It's time for versioned documentation and better samples!

If you want to help out with this please comment on this issue and I'll add you as a contributor to my personal repository.

See following for inspiration:

* http://jacobian.org/writing/what-to-write/
* http://jacobian.org/writing/technical-style/
* http://jacobian.org/writing/editors/

See following for collection of projects with beautiful documentation: https://github.com/PharkMillups/beautiful-docs for inspiration

## Operations:
- [x] Use Readthedocs.org to automatically build ReactiveUI documentation as it supports versioned documentation via branches.
- [x] Resolve https://github.com/tomchristie/mkdocs/issues/238
- [ ] CNAME created for [docs.reactiveui.net pointed towards readthedocs](http://read-the-docs.readthedocs.org/en/latest/alternate_domains.html).
- [x] Determine what to use and how to automatically build class documentation.
- [ ] Documentation Theme
- [x] Structure of documentation defined.
- [ ] Migrate existing docs/* across.
- [ ] Clean existing docs/* to drop all stale Rx4 and Rx5 references.

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## Ideas for samples?
- [ ] Sub-classing of UserError
- [ ] Handling subclassed UserError Inline or via Global Registration
- [ ] Using IRecoveryCommand and RecoveryOptionResult? "I have a LoginCommand which internally does an http request to my server. Depending on the Response (200, 401, <5xx and timeouts>), I'd like to either (200 -> dismiss login dialog / 401 -> show "bad credentials" alert message ("OK") / <5xx and timenouts> -> show "Error" alert message ("Cancel", "Retry"). I thought throwing an UserError for 401 and   <5xx and timenouts> with a retry command would make sense, but I'm not sure how to  use the built-in Recovery stuff in ReactiveUI."
- [ ] General Navigation - WPF and WinForms
- [ ] Xamarin Forms - Navigation, Binding on all platforms.
- [ ] Suspension Host on all platforms
- [ ] Service Location
- [ ] Caching using Akavache
- [ ] Integrating ModernHTTPClient with Fusillade and Refit
- [ ] Using Splat/IBitmap x-platform within the PCL and value converters on each platform to display images.
- [ ] Reactive Dialogs. PCL Dialog Service -> iOS/Android impl which is observable based on which button pressed. Yes, No, Cancel.
- [ ] Example of a custom RoutedViewHost  (say iOS tab menu or navigation stack)
Any tips for testing out these docs locally?

This is what I get when running from the root of the repository:

> make docs
make: Nothing to be done for `docs'.