Add a style guide for developers documentation

We have too many libraries, written by too many people, to leave the documentation up to whoever decides to write it. Let's write down some guideline so that people writing an API reference can at least not make up rules as they go.

Missing style sections:

• Classes, interfaces, and structures
• Methods and functions
• Properties and signals
• Enumerations and flags
• Code samples
• Deprecations
• Command-line syntax
• UI elements