1020 document compositionoptionsuseregiondirtyrectclipping being disabled by default

[luke-whos-here]

This PR adds documentation to cover changes to CompositionOptions.UseRegionDirtyRectClipping introduced by AvaloniaUI/Avalonia#21553.

• Adds a new section to the Performance Optimization guide to explain the impact of region dirty rect clipping, with instructions to enable the option if desired, and a note on setting max dirty rects.
• Adds a new section to the Embedded Linux platform guide to call out that the option is beneficial to that platform, with instructions to enable the option.
• Add crosslinks between the relevant sections.
• Opportunistic general edits and updates on the Performance Optimization page, which was probably overdue for a revision anyway.

[Copilot]

Pull request overview

This PR updates Avalonia documentation to explain the Avalonia 12.1 change where CompositionOptions.UseRegionDirtyRectClipping is disabled by default, and to guide users on enabling it (especially for embedded Linux scenarios).

Changes:

• Adds a new embedded Linux section explaining when/why to enable region dirty-rect clipping and links back to performance guidance.
• Adds a new “Region dirty rect clipping” subsection to the performance optimization guide, including MaxDirtyRects notes and cross-links.
• Refreshes several existing performance guide sections for clarity and structure.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 9 comments.

File	Description
docs/platform-specific-guides/embedded-linux/embedded-linux.md	Adds a rendering performance section focused on enabling region dirty-rect clipping for embedded Linux, plus cross-links and frontmatter metadata.
docs/app-development/performance.md	Adds/updates performance guidance including a new region dirty-rect clipping section and multiple editorial improvements across the page.

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[MrJul]

For clarity, the option was and is still null by default, which means "Avalonia chooses what's best", so we can change the behavior in future releases if we deem it more optimal (like we just did). In 12.1, Avalonia chooses that false is now the best.

(As long as we keep the docs up-to-date, I'm not sure that difference is worth explaining as this might confuse the user: after all, it's definitely "false by default" as far as they're concerned.)