Develop hook docs

[kodaline]

Add

@plugin documentation including:

• settings schema
• settings model
• load settings
• save settings Todo: improve load/save example (?)

relates to issue #101

[kodaline]

@nicola-corbellini no rush to merge it, let me know what you think the reference for the settings model will work well once we have the API reference for it 😄

[nicola-corbellini]

Thank you!