Create a developers guide

[tim-vd-aardweg]

What is the need for this task.
We need documentation for some common developers procedures/tasks. This documentation can also be useful for reviews.

What is the task?
Create a document where we can list guides and procedures for developers. Content can for example include:

• Updating the __init__.py files if we change the public API.
• Updating the dhydro_support_hydrolib-core.xlsx if new functionalities have been added.
  - How to test new models. (Should be done when we have refactored the parsers and serializers)
  -- When do we use files for test data and when do we use in-memory data.

[priscavdsluis]

#16 can probably be closed with this issue.

[tim-vd-aardweg]

As discussed during the standup:

• I have removed the developers guide from the online documentation. It is now only a document that can be found in the repo.
• The initial contents will remain as is. When we overhaul the documentation, we can see if items from the How to contribute page can be moved to the developers guide.
• When we pick up the refactoring of the parsers and serializers (Refactor the parsers and serializers #503), we should add something about how we want to test parsers and serializers.