Add documentation and tests

Hi @Harlor (Maintainer) and all others,

thank you for creating this module from the GitHub Code: https://github.com/harlor/ldap_profile/issues/1#issuecomment-789659387
I've just added the dev branch as default branch. We should now discuss what steps are needed to create the first stable release.

TODO:

• Add clear description & screenshots to the module page
• Write tests
• Define compatibility with LDAP module versions
• Test Drupal 9 compatibility
• ...?