[google_maps_flutter] Federate READMEs

[stuartmorgan-g]

Moves the platform-specific setup logic and discussion from the main README to the platform READMEs, per our current best practice.

As part of the move, I simplified the discussion of the iOS implementation package choice somewhat, since I realized there was too much explanation of why things are the way they are, rather than on the steps a client should take.

There's a repeated example code block across the iOS packages now, but I think duplicating that is much better than being inconsistent about where setup info is (or worse, pointing them to one implementation package that's not the same as the one they are using).

Part of flutter/flutter#182772

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩²

[gemini-code-assist]

Code Review

This pull request updates the documentation across multiple Google Maps Flutter packages to centralize platform-specific setup instructions. Instead of duplicating detailed setup steps in the main package README, it now links directly to the platform-specific implementation packages (Android, iOS, and Web) and adds the corresponding setup guides to their respective README files. Feedback is provided regarding the formatting of the documentation, including correcting the indentation of nested code blocks in the Web README, aligning XML attributes in the Android README, and suggesting direct anchor links for the iOS setup sections.