Move hardcoded Aapt2Compile warning string to Resources.resx for localization

[Copilot]

Aapt2Compile emitted a hardcoded, uncoded warning when a resource directory no longer existed. This moves the message into the localization resources and follows the established coded-warning pattern.

Changes

• Add localized resource XA4323: Ignoring directory '{0}' as it does not exist!
• Emit the warning with Log.LogCodedWarning("XA4323", Properties.Resources.XA4323, fileOrDirectory)
• Document XA4323 and add it to the warning index and documentation TOC
• Update Copilot and Android reviewer instructions to require coded warnings/errors, localized resources, and message documentation

The warning text remains unchanged; it now includes the XA4323 code and can be localized.

Fixes #12226

[Copilot]

Pull request overview

Moves a hardcoded English warning message in the Aapt2Compile MSBuild task onto the existing Properties.Resources localization pipeline, aligning warning output with the repo’s localized resource conventions (per issue #12226).

Changes:

• Add a new Aapt2IgnoringDirectory string resource (with {0} placeholder and comment) to the main English Resources.resx.
• Update Resources.Designer.cs to expose the new strongly-typed resource property.
• Replace the interpolated warning literal in Aapt2Compile with a resource-based formatted warning message.

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/Xamarin.Android.Build.Tasks/Tasks/Aapt2Compile.cs	Switches the warning emission from an English literal to a localized resource string.
src/Xamarin.Android.Build.Tasks/Properties/Resources.resx	Adds the new Aapt2IgnoringDirectory resource entry with placeholder and translator comment.
src/Xamarin.Android.Build.Tasks/Properties/Resources.Designer.cs	Adds the generated accessor property for the new resource key.

Files not reviewed (1)

• src/Xamarin.Android.Build.Tasks/Properties/Resources.Designer.cs: Generated file