Fix broken links and reference AndroidX (#484)
Fix broken links and reference AndroidX (#484)

- Use class/namespace names to display links (easier to browse).
- Reference the API namespace url instead of each individual class (less future maintenance required). The exception is the classes included in the core ReactiveUI project because those don't have a sub-namespace.
reactiveui/website,reactiveui/website,reactiveui/website,reactiveui/website