docs: add Avalonia XAML Playground recipe and update .NET app guide

[Rinconjr]

add Avalonia XAML Playground recipe and update .NET app guide

Adds a 'Craft a .NET Avalonia app' section to the 'Craft a .NET app' how-to, covering platforms, the dotnet v2 part, rendering dependencies, the dotnet extension + desktop plugs and lint-ignore config. Includes a new example recipe, and wordlist additions.

• I've followed the contribution guidelines.
• I've signed the CLA.
• I've successfully run make lint && make test.
• I've added or updated any relevant documentation.
• In documents I changed, I added a meta description if one was missing.
• I've updated the relevant release notes.

[Copilot]

Pull request overview

Adds an Avalonia-focused recipe to the existing “Craft a .NET app” how-to, extending Snapcraft’s .NET documentation to cover desktop-specific considerations on core24+ (platform constraints, rendering deps, extensions, plugs, and linter ignores).

Changes:

• Expands the .NET how-to with a new “Craft a .NET Avalonia app” section, including guidance for core24+ dotnet v2 usage and desktop interfaces.
• Adds a new example snapcraft.yaml recipe for Avalonia XamlPlayground.
• Updates the docs custom wordlist to accommodate new terminology.

Documentation review pipeline (skill-orchestrated):

• ✓ Stage 1 complete: documentation-build incomplete (RTD/Sphinx artifacts detected; build execution not available in this environment). Referenced literalinclude files were verified to exist.
• ✓ Stage 2 complete: Structure mapped (Sphinx under docs/ with Diátaxis-style top-level directories like how-to/, reference/, explanation/, tutorials/).
• ✓ Stage 3 complete: documentation-diataxis found no misalignment for the updated how-to page (still fits a goal-oriented how-to).
• ✓ Stage 4 complete: documentation-structure found no structural violations in the changed page.
• ✓ Stage 5 complete: documentation-verify found no accuracy issues for key claims (e.g., dotnet-version, dotnet-configuration, and dotnet10 are evidenced in repo docs/tests).
• ✓ Stage 6 complete: documentation-style found no style violations in the changed reST content.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
docs/how-to/integrations/craft-a-dotnet-app.rst	Adds an Avalonia desktop app flow for core24+ alongside the existing CLI-focused .NET guidance.
docs/how-to/code/integrations/example-dotnet-avalonia-recipe.yaml	Introduces an Avalonia XamlPlayground snapcraft.yaml recipe illustrating dotnet10 extension usage, rendering deps staging, and linter ignores.
docs/.custom_wordlist.txt	Adds Avalonia/XAML-related terms to reduce spelling false positives in docs CI.

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[mr-cal]

I'm going to defer to @canonical/starcraft-authors for the first pass, as they may have opinions on the location/structure that should preceed a techincal review from me.

[jahn-junior]

Thanks for this docs contribution! This all seems pretty sound and feels consistent with the current content. Nearly all of my suggestions are minor language tweaks. Feel free to ask for clarification or rationale on any of them.

[Rinconjr]

Hey @jahn-junior, thank you for your comments, I've addressed all of them. I agree with all of them since they improve the general quality of the docs. Please let me know what you think about the updated docs 😄