misc: Cleanup and Prep for Arduino IDE

[sleepdefic1t]

Proposed changes

This PR is cleanup prep for upcoming Arduino IDE branch.
It removes unnecessary files and moves others to a cleaner more Arduino-friendly layout.
It also properly configures ./library.json to export packages correctly for external library managers.

The following has been changed:

• changed package export settings in ./library.json.
• removed submodule from cmake_example.
• moved ./docs to ./extras/docs.
• moved ./src/stl to ./src/lib/stl.
• removed .appveyor.yml.
• removed ./CMakeSettings.json.

All tests pass locally:

• macOS - CMake.
• macOS - PlatformIO.
• macOS - Arduino IDE

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (improve a current implementation without adding a new feature or fixing a bug)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Build (changes that affect the build system)
• Docs (documentation only changes)
• Test (adding missing tests or fixing existing tests)
• Other... Please describe:

Checklist

• I have read the CONTRIBUTING documentation
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[ghost]

@air1one @faustbrian - please review this in the next few days. Be sure to explicitly select labels so I know what's going on.

If no reviewer appears after a week, a reminder will be sent out.

[ghost]

@sleepdefic1t Your pull request doesn't follow our contribution guidelines. Please review and correct it.