Improve developer documentation of submission state and component configuration

[Viicos]

While working on the Objects API registration, I sometimes found it hard to understand how the submission state should be loaded. Here's a couple things I noted:

• The SubmissionValueVariablesState (available through submission.load_submission_value_variables_state) has 11 public methods/attributes, making it hard to find out the right one to use. Naming is sometimes ambiguous (e.g. get_data). None of them have docstrings.
• There seem to be a distinction between "saved variables" (with a PK) and variables not persisted, but it is unclear what is the use case for this.
• The SubmissionValueVariable name is confusing: it should probably be SubmissionVariableValue instead. However not sure if it is possible to rename it.

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I had the same trouble with components configuration:

• The recommended way to access the component configuration is through the FormioConfigurationWrapper class, but is also missing docstrings.
• While I understand a component is following the {"type": "...", "key": "..."} structure, it is still unclear to me how that plays with repeating groups and/or nested components. This relates to the FormioData structure.

For this, I would really like having some kind of introduction in the docs to the formio structure, how it is integrated in Open forms and how we manage to extend it.

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I would also really like having some kind of "cheat sheet" in docs, stating the recommended ways of dealing with the submission state, the formio configuration wrapper, etc

[sergei-maertens]

I'd also like a critical review of these service layers - they have grown beyond their initial scope because of certain requirements.

Let's figure out what developers need and how we can provide this while keeping the implementation evolvable.

[joeribekker]

Refinement: We focus first on documenting existing behaviour and make a list of possible improvements. Focus on the intent of the service layers. Timeboxed on a day + team discussion.

@Viicos starts with a rough draft so we can shoot at it.