Remove unsupported directory syntax from pubspec shaders section

[jesskuras]

Description of what this PR is changing or adding, and why:

Updates the shaders field section on the Flutter pubspec options page (/tools/pubspec#shaders):

• Removes the path_to_directory syntax and directory example, clarifying that each .frag shader file must be listed individually (since flutter_tools only parses individual file paths under flutter.shaders).
• Removes the suggestion to add shaders under the assets field (since flutter_tools requires .frag shaders to be declared under shaders to compile them with impellerc).
• Notes support for platforms alongside flavors and transformers, and links to the Writing and using fragment shaders guide.

Issues fixed by this PR (if any):

Fixes #13972

PRs or commits this PR depends on (if any):

None

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[flutter-website-bot]

Staged preview of the updated docs.flutter.dev site (updated for commit ece52d3):

https://flutter-docs-prod--docs-pr13974-pubspec-shaders-files-m2cueur6.web.app

[gemini-code-assist]

Code Review

This pull request updates the documentation for the shaders field in pubspec.yaml to clarify that only individual file paths are supported and that shaders should not be listed under the assets field. The review feedback correctly points out that shaders only support flavors and do not support platforms or transformers, suggesting a correction to the text.

[parlough]

Thanks for fixing this and the nearby clean up. Looks good to me!