[flutter_local_notifications] Refactor the README into platform-specific guides

[Levi-Lesches]

Based on the work in #2308, this PR will split the README into platform-specific guides and topics (that can then be hosted on another website besides github, if desired), and leave the main README for Dart-specific details. Specifically, my goal is to:

• split the main README into different markdown files, at least one per platform
• reorganize the contents on each page (eg, group all Gradle changes together)
• rewrite the docs for clarity
• include more links and examples
• remove info that only applies to versions of Flutter or platforms this plugin no longer supports

Progress:

• Android setup guide
• iOS setup guide
• Windows setup guide
• General usage guide
• Android usage guide
• Darwin usage guide
• Windows usage guide
• Linux usage guide
• Main README cleanup (should only have: links to all other guides, feature matrix)

My plan is to only generate a table of contents on the main README, as GitHub will automatically provide a ToC while Pub does not (but might be planning to)

cc @davetrollope if you're interested

[Levi-Lesches]

Just finished the Android page, I will try to use a similar style for the other platforms.