docs: improve code coverage document

Fixes: #6507

This patch improves the code coverage use case for integration
tests and unit tests.

Signed-off-by: Songqian Li <sionli@tencent.com>

docs: add notes on collecting coverage data from in-tree tests

Signed-off-by: Wei Liu <liuwe@microsoft.com>

docs: add a document for collecting coverage data

Signed-off-by: Wei Liu <liuwe@microsoft.com>