Document test coverage status

[chengweih001]

Currently the Web Bluetooth manual test coverage is listed in the internal document. We can consider to move the test coverage in the internal document to https://github.com/WebBluetoothCG/manual-tests/blob/main/README.md or create a new one called ROADMAP.md or TEST_COVERAGE.md. This way the information is easier to discover when browsing the test code files.

For each row entry in the test coverage list, in addition to show status and the API like currently we have in the internal document.

Status	Function/Attribute
✅	BluetoothCharacteristicProperties.read

We can consider add another column to indicate that can be tested in what test directory in the repo or point to the test page (ex: https://webbluetoothcg.github.io/manual-tests/characteristic_read/)

Status	Function/Attribute	Test link
✅	BluetoothCharacteristicProperties.read	https://webbluetoothcg.github.io/manual-tests/characteristic_read/