Improve Controls Core API docs 2 (dotnet#33243)

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]](diffhunk://#diff-c044c2b011c27c4ffdd80e3c5c18bc2d220916e74493284c4adc46cb42654c80L9-R14)
[[2]](diffhunk://#diff-c044c2b011c27c4ffdd80e3c5c18bc2d220916e74493284c4adc46cb42654c80L21-R47)
* Added comprehensive XML documentation comments to the `Border` class
and its `Content` and `Padding` properties, describing their purpose and
usage.
[[1]](diffhunk://#diff-356d119e99e7b0473adc38b35839aff1ab325ba60796b0f4774e549b47400d4aR12-R18)
[[2]](diffhunk://#diff-356d119e99e7b0473adc38b35839aff1ab325ba60796b0f4774e549b47400d4aR45-R56)

**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.

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Co-authored-by: Shane Neuville <[email protected]>

Author: jfversluis