Add FAQ entry about white labeling

[epalchenko]

PR description

This PR adds a new frequently asked question about the ThingsBoard logo, along with the corresponding answer.

Link checker

The links will be checked by the build agent automatically once you create or update your PR.

You can use the following command to check the broken links locally.

docker run --rm -it --network=host --name=linkchecker ghcr.io/linkchecker/linkchecker --check-extern --no-warnings http://0.0.0.0:4000/