documentation of idmanager box

[stefanlack]

• Getting Started guide (https://www.opendevstack.org/ods-documentation/common/latest/getting-started/local-install.html) leads through the first local installation.
• In the guide, only installation with crowd authentication is described.
• oauth2 authentication was added in provisioning app

Implemented but not documented:

• a idmanager box was added via commits https://github.com/opendevstack/ods-core/commit/4a03051e02015b16fe235f5a58a31427c9974d02

Not implemented and not documented:

• Setup of local installation by using_oauth2_ instead of crowd, skip crowd completely

[stefanlack]

I will document this together with the changes of https://github.com/opendevstack/ods-core/pull/185