[cupertino_icons] Update README to link to API docs

[jmagman]

1. Update 0.1.3 glyph map from broken https://raw.githubusercontent.com/flutter/cupertino_icons/master/map.png to updated https://raw.githubusercontent.com/flutter/packages/master/third_party/packages/cupertino_icons/map.png.
2. Replace dead link https://flutter.github.io/cupertino_icons to instead link to the API docs.

Package README/pub documentation part of flutter/flutter#88754.

Pre-launch Checklist

• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide].
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt.
• I updated pubspec.yaml with an appropriate new version according to the [pub versioning philosophy].
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I signed the [CLA].
• All existing and new tests are passing.

[guidezpl]

Thanks for doing this! LGTM

[stuartmorgan]

LGTM, thanks!