Add tvOS documentation and revise quickstart instructions

[DellaBitta]

A revision of the Quickstart App instructions for tvOS and general clarity.

Changes were made to:

• Normalize use of text formatting:
  • UI elements are now always represented as bold text.
  • File names and strings are now in code blocks.
  • Previously the documentation switched back and forth between different conventions.
• Normalize instructions on how to open the testapp and the Main Scene. Some quickstarts had varying text here, some lacked any sort of documentation on the subject.
• Ensure each platform has explicit setup steps for both iOS and Android.
  • Some testapps assumed that the user had completed steps in the desktop setup.
• Update docs on Unity Editor and Xcode minimum versions.
• Update iOS section to be applicable to tvOS, too.
• Add a comment that the testapp's UI elements don't respond to clicks on tvOS.
• Update instructions for the Project <-> App association flow in the Firebase console.
  • click the Unity icon instead of assuming the user clicks the Android / iOS icon.
  • Update "iOS Bundle Id" to "Apple Bundle Id", which is how it is now presented in the Firebase console.
• Remove the .net v3.5 .unitypackage instructions.
• Fix punctuation.

[DellaBitta]

Yes, Firestore has their own readme that their team maintains and doesn't have the same information (for better or worse) as our other readmes.