docs: Add dot shorthands guide to Flutter concepts

[lamek]

Adds a guide on Dart shorthands to the flutter docs.

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  of 80 characters or fewer.

[gemini-code-assist]

Code Review

This pull request adds a helpful guide on using dot shorthands in Flutter. The documentation is clear and provides good examples. I've pointed out a minor grammatical error in the new markdown file. I also noticed that the new page is placed in the 'Performance & optimization' section of the navigation, which seems incorrect. I've recommended moving it to a more suitable section like 'Flutter concepts' to improve the documentation structure.

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit 66cb2dc):

https://flutter-docs-prod--pr13180-clean-dot-shorthands-axell281.web.app

[lamek]

CC @kallentu