document all configuration options available in acre.yaml

[Diaphteiros]

The user has to provide an acre.yaml file containing the configuration. Most relevant parts are documented here. However, some components support additional options or specific edge cases that require further configuration - which is partly not documented at all.

To not overstrain new users with a huge README, these features and their corresponding configuration options should be documented in an extra document (or multiple documents).

Structure of the documentation can be discussed here.

[Diaphteiros]

It is probably a good idea to document this feature-wise, meaning each feature gets its own documentation page, also containing the information on how to configure it in the acre.yaml.

Documenting the acre.yaml with all configuration options and what they do in one single file (like it is in the README linked above) is likely going to result in an extremely long and hard-to-understand file.

[Diaphteiros]

Several "hidden" configuration options have been documented by now. You can see an overview here: https://github.com/gardener/garden-setup/blob/master/docs/extended/index.md

However, some are still missing, e.g. the connectors option for identity. The extended configuration index linked above could also be improved.