docs: update Genkit Dart quickstart links to overview page and simplify image component syntax

[abdallahshaban557]

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

Update the getting started links for Genkit

Issues fixed by this PR (if any):

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

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 updates several links referencing the Genkit Dart quickstart from the old get-started path to the new overview path across documentation and UI files, alongside minor formatting cleanups for image elements. The review feedback suggests updating another missed hardcoded link in create-with-ai.md to use the reference link, and removing trailing slashes from the newly updated URLs in ai_page.dart for consistency.

[flutter-website-bot]

Staged preview of the updated docs.flutter.dev site (updated for commit 4dc117c):

https://flutter-docs-prod--docs-pr13482-update-genkit-links-h6gf0qin.web.app

[flutter-website-bot]

Staged preview of the updated flutter.dev site (updated for commit 4dc117c):

https://flutter-dev-230821--www-pr13482-update-genkit-links-n13r2oe3.web.app