docs: documentation quality improvements

[w-ahmad]

Summary

Polishes the documentation site based on a thorough review of https://w-ahmad.dev/WinUI.TableView/.

Changes

Fix outdated AdvancedCollectionView references

• AdvancedCollectionView was replaced with a custom internal collection view back in v1.3.0. Updated all references in overview.md, sorting.md, filtering.md, binding-data.md, performance.md, and migration-wct.md to reflect the current implementation.
• Removed the stale using CommunityToolkit.WinUI.Collections import from the filtering code sample.

Polish Getting Started wording

• Fixed "Create a New WinUI3" → "Create a new WinUI project"
• Normalized all step headings to sentence case across both Getting Started pages.

Add Uno Platform notes (getting-started-with-uno.md)

• New Uno Platform notes section covering: WebAssembly, Desktop (Skia), Android/iOS, binding behavior, keyboard/focus differences, and known limitations per target.

Add column type decision table (column-types.md)

• New "Choosing a column type" section near the top with a reference table mapping common needs to the recommended column type.

Update WinUI branding

• Prefer WinUI over WinUI 3 in general descriptions across index.md, overview.md, performance.md, accessibility.md, and aot-compatibility.md.
• Kept "WinUI 3" where technically specific (e.g. AOT applies to the Windows App SDK / WinUI 3 target specifically).

Fix missing section heading in sorting.md

• Restored the missing ## Sorting programmatically heading before the SortDescriptions code example.

Not included

• Recipes page — deferred until screenshots are available for the conditional styling and row details recipes.