doc: coverage: Add Twister `--coverage-basedir` use case

Add an example of using Twister for an 'out-of-tree' application's
code coverage report generation with `--coverage-basedir` option,
and a note about `gcovr` tool possible problems with symlinks.

Fixes: #83764

Signed-off-by: Dmitrii Golovanov <dmitrii.golovanov@intel.com>