Add API documentation skill and improve API reference docs

[mattleibow]

Description of Change

This PR adds Copilot skills for documentation workflows and significantly improves public API reference documentation.

Copilot Skills (Repository Documentation)

• api-docs skill: Guides AI assistants in writing and reviewing XML API documentation following .NET guidelines. Includes:
• XML documentation patterns and syntax reference
• Review checklist with severity classifications (Critical/Important/Minor)
• Quick grep commands for finding common issues
• XML validation workflows
• skill-creator skill: Provides guidance for creating new Copilot skills

Public API Reference Documentation (docs submodule - 548 files)

• Fixed spelling errors (e.g., "Represets" → "Represents")
• Fixed invalid <paramref> references (e.g., buffer. → buffer)
• Added missing periods to summary, param, and returns tags
• Added missing <see langword> tags for boolean values
• Fixed empty parameter documentation
• Updated framework indices for SkiaSharp 3.116/3.119, HarfBuzzSharp 8.3, MAUI, Skottie, SceneGraph, Resources
• Removed obsolete 1.x framework indices

Bugs Fixed

None.

API Changes

None.

Behavioral Changes

None.

Required skia PR

None.

PR Checklist

• Has tests (if omitted, state reason in description) - Documentation changes only
• Rebased on top of main at time of PR
• Merged related skia PRs
• Changes adhere to coding standard

[github-actions]

Triage Summary

No labels will be applied as the issue lacks sufficient specific information necessary for categorization.

This issue is not a regression since it does not contain enough details to ascertain any previous working state or functionality.

Additional remarks:

• The issue content primarily resembles a template and does not mention relevant areas like build issues or specific libraries (HarfBuzzSharp, SkiaSharp).
• The description does not provide enough context regarding changes, bugs, or performance aspects, impairing accurate labeling.
• There is no mention of the operating system or platform, making it impossible to categorize appropriately.
• The lack of technical context or detailed information prevents any meaningful label assignment.

Detailed Summary and Actions

Summary of the triage:

• No labels will be applied due to insufficient specific information regarding the issue.
• The issue description resembles a template and lacks details on various potential categories.
• There are no identifiable changes or problems that warrant a label assignment.

Summary of the actions that will be performed:

Action	Item	Description
No Action	-	Insufficient details prevent any action or categorization.

This entire triage process was automated by AI and mistakes may have been made. Please let us know so we can continue to improve.