Clarify guidance on loading untrusted XAML, BAML, and XPS content (#2261

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* Clarify guidance on loading untrusted XAML, BAML, and XPS content

Add guidance that loading untrusted markup is equivalent to running untrusted code and that restrictive or allow-list readers are a hardening measure, not a security sandbox. Update the XAML Services security article with platform-agnostic guidance and add a WPF-specific section covering text XAML, BAML, and XPS/fixed-document loading, with cross-links between the two articles.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot-Session: 0145ac7e-a38f-4948-9bde-134f90cff73f

* Refine wording for XPS and fixed-document types

Change 'Document containers' to 'Document types' to avoid implying that DocumentReference and PageContent are container formats. Guidance is unchanged.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot-Session: 0145ac7e-a38f-4948-9bde-134f90cff73f