Update OpenXML MS documentation

[farzonl]

Describe the bug
After this commit a number of APIs were removed:
4260711
For example
GetPartsCountOfType was removed but the documentation doesn't indicated deprectated or removed in the applies to section.
https://learn.microsoft.com/en-us/dotnet/api/documentformat.openxml.packaging.openxmlpartcontainer.getpartscountoftype?view=openxml-2.8.1

infact the documentation hasn't been updated since 2.8.1 and these apis lived on through 2.20.0
/~https://github.com/dotnet/Open-XML-SDK/compare/v2.20.0...v3.0.0-beta1.patch

This is particularly unpleasant because the example code in the documentation is also incorrect: https://learn.microsoft.com/en-us/office/open-xml/how-to-insert-a-comment-into-a-word-processing-document

It does appear the documentation is hosted on github. Can you please update the documentation? While I muddled through, porting my app to the latest sdk, It would be good for future users of the sdk to have a single source of truth.

[twsouthwick]

This is underway! We had kind of left those docs unattended for a number of years... working to bring them back up to shape for the v3.0 release :) I'm going to close this issue in favor of this tracking issue: OfficeDev/open-xml-docs#159. Please add any comments there for additional things that you may notice that need to be updated.

[twsouthwick]

FYI the api updates have been completed and the process is now known for updating for future releases