Create OCM Library developer documentation

[In-Ko]

Description
The library needs a good developer documentation, explaining how it should be used for the most common usecases. There must be example usages for these usecases, to enable developers using the library in an easy to follow approach. Possibly, this documentation could show how the library is used in the OCM CLI.

This documentation needs to be available also on the ocm.software website, similar to the OCM client documentation.

[In-Ko]

Another open source project (Landscaper: https://github.com/gardener/landscaper), intents to make use of the go libraries. At its current state (complexity and especially missing developer documentation), it seems very hard to do so.

This is why some Landscaper developers already looked into the library and are in preparation for a first iteration of a documentation, outlining the go library structure and also some details of certain packages.

[In-Ko]

The open-component-model/ocm#367 is a start to provide an overview of the ocm library, for developers.

[fabianburth]

The documentation process in open-component-model/ocm#367 is currently on-hold due to considerable reworks of the ocm-lib that affect the basic concepts explained in the documentation.

Generally, the following tasks will have to be adressed:

• Rework and improve the deprecated parts of the documentation
• Incorporate feedback (make explanations even more clear and enrich with examples)

[morri-son]

duplicate of #124