docs: Add dot shorthands guide to Flutter concepts

[lamek]

Adds a guide on Dart shorthands to the flutter docs.

Presubmit checklist

• If you are unwilling, or unable, to sign the CLA, even for a tiny, one-word PR, please file an issue instead of a PR.
• If this PR is not meant to land until a future stable release, mark it as draft with an explanation.
• This PR follows the Google Developer Documentation Style Guidelines—for example, it doesn't use i.e. or e.g., and it avoids I and we (first-person pronouns).
• This PR uses semantic line breaks
  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 7671792):

https://flutter-docs-prod--pr13180-clean-dot-shorthands-2trht05o.web.app

[lamek]

CC @kallentu

[sfshaza2]

lgtm! Just a couple minor edits. I wasn't going to bother with adding a page for this, but this is a great way to handle it!