[DOC] Write developer documentation for type hints including soft dependencies.

[MatthewMiddlehurst]

Describe the issue linked to the documentation

Developer documentation does not sufficiently include guidance for adding type hints, espceically in situations where soft dependencies are required.

Suggest a potential alternative/fix

Add a new page in docs/developer_guide/ on adding type hints. PyODAdapter is a good example of type hints with soft dependencies, following PEP 563.