Improve Controls Core API docs 2

[jfversluis]

This pull request updates the documentation comments for two core controls, ActivityIndicator and Border, by replacing <include> tags with explicit XML documentation comments. This makes the documentation self-contained and easier to maintain. Additionally, the PR removes the legacy XML documentation files for ContentView and ProgressBar, likely as part of a broader move away from external XML doc files.

Documentation improvements for controls:

• Replaced <include> tags with explicit XML documentation comments for the ActivityIndicator class and its key properties and constructor, providing clear inline summaries and remarks. [1] [2]
• Added comprehensive XML documentation comments to the Border class and its Content and Padding properties, describing their purpose and usage. [1] [2]

Removal of legacy documentation files:

• Deleted the XML documentation file for ContentView, removing all related type and member documentation.
• Deleted the XML documentation file for ProgressBar, removing all related type and member documentation.