component_guide: Update import-debian-package documentation

import-debian-package script now runs all the steps automatatically. The
recently added --push-remote option allows creating and pushing the
imported package to apertis remote gitlab instance.

The following steps are now automatized:
* package folder is created and import process is performed in there.
* remote repo is created if it doesn't exist.
* remote branches and tags are pushed.
* gitlab settings are applied to remote repo.
* CI pipeline is triggered on all downstream branches.

The previously manual steps (performed now by the script) are kept
documented in the import-debian-package repository.

Signed-off-by: Ariel D'Alessandro <ariel.dalessandro@collabora.com>