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.

[Copilot]

Pull request overview

This pull request improves the XML documentation for several core .NET MAUI controls by replacing external <include> file references with explicit inline documentation comments. This makes the documentation self-contained and easier to maintain directly within the source code.

Key changes:

• Replaces <include> tags with explicit XML documentation for 10+ core controls
• Adds comprehensive summaries, value descriptions, and remarks for class and member documentation
• Removes legacy XML documentation files (ContentView.xml and ProgressBar.xml) that are no longer needed
• Fixes incomplete documentation fragments and a typo

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
ActivityIndicator.cs	Replaced include tags with explicit documentation for class, constructor, Color, and IsRunning properties
Border.cs	Added comprehensive documentation for class, properties (Content, Padding, Stroke*, Shape), and methods with summaries and remarks
TimePicker.cs	Replaced include tags with explicit documentation for class, constructor, and all properties (Format, TextColor, CharacterSpacing, Time, Font*, IsOpen)
ProgressBar.cs	Replaced include tags with explicit documentation for class, constructor, properties (ProgressColor, Progress), and ProgressTo method
Picker.cs	Enhanced documentation for class and replaced include tags for properties (CharacterSpacing, TitleColor, HorizontalTextAlignment, VerticalTextAlignment, IsOpen)
Label.cs	Replaced include tags for TextTransform, CharacterSpacing, and Padding properties
Image.cs	Enhanced class-level documentation and replaced include tags for Aspect, IsLoading, IsOpaque, and IsAnimationPlaying properties, removing incomplete remarks
DatePicker.cs	Enhanced class-level documentation and replaced include tags for CharacterSpacing, FontSize, and IsOpen properties
ContentView.cs	Replaced include tags with explicit documentation for class, Content property, with detailed remarks
BoxView.cs	Added documentation for OnMeasure, OnPropertyChanged, and PathForBounds methods, fixing a typo in the process
ProgressBar.xml	Deleted legacy XML documentation file (249 lines)
ContentView.xml	Deleted legacy XML documentation file (137 lines)

Comments suppressed due to low confidence (1)

src/Controls/src/Core/Border/Border.cs:60

• The Padding property documentation should include a value tag describing the type and default value. Add: /// <value>A <see cref="Thickness"/> structure representing the padding around the content. The default is <see cref="Thickness.Zero"/>.</value>

		/// <summary>
		/// Gets or sets the padding inside the border. This is a bindable property.
		/// </summary>
		public Thickness Padding
		{
			get => (Thickness)GetValue(PaddingElement.PaddingProperty);
			set => SetValue(PaddingElement.PaddingProperty, value);