Document our documentation process/workflow/guidelines

[rolfbjarne]

Ref: #14372, step 6

Write up a doc about how to write documentation, similar to what this is for MAUI, but specifically for our use case: dotnet/maui@main/docs/CodeDocumentationGuidelines.md

• Style guide: https://github.com/dotnet/dotnet-api-docs/wiki
• Recommended XML tags: https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/xmldoc/recommended-tags