[lite] platform switcher in CBL docs

First comment:

Currently, CBL docs cover all the platforms on the same page. This adds context for users that are usually only interested in 1 platform:

• iOS/macOS/tvOS: ObjC, Swift
• Android/Pure Java/IoT: Java, Kotlin
• WPF/UWP/Xamarin: C#

In 2.0, a CBL documentation page should only show the content for the platform that the user is after.

Todos:

• [ ] Show list of platforms below the TOC.
• [ ] Use Liquid tags to stripe the content per platform.
• [ ] When a platform is selected, update the content on the page.
• [ ] Persist the selected item in a cookie.
• [ ] When a platform is selected, add it to the querystring `?language=java`.
• [ ] Make the list of platforms scroll with the page so users can switch platform half way down the page.

see other comments: https://github.com/couchbaselabs/couchbase-mobile-portal/issues/535