[file_selector] Include the missing GetDirectoryPath method example and table of supported platforms

[VanesaOshiro]

Adds the following:

• Table of supported platforms
• The missing GetDirectoryPath method example

Table of supported platforms

GetDirectoryPath method example

Related issue:

• [file_selector] Documentation enhancements #111753

Pre-launch Checklist

• 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 relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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[stuartmorgan-g]

Thanks for the submission! See inline for comments.

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat (don't just cc him here, he won't see it! He's on Discord!).

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[VanesaOshiro]

Thanks for the submission! See inline for comments.

Hello Stuart! Thanks for your feedback, I've made the necessary changes, please feel free to review again. On the other hand, the bot has assigned the "needs tests" label, but I've only updated the documentation, so some hours ago I sent a request for test exemption.

[Hixie]

test-exempt: documentation

[stuartmorgan-g]

A few more small nits, but otherwise looks good!

[stuartmorgan-g]

@cbracken for secondary review

[VanesaOshiro]

A few more small nits, but otherwise looks good!

We've applied the suggested changes, thanks!

[cbracken]