SemConv guidance: document how to define events

[lmolkova]

See https://github.com/open-telemetry/semantic-conventions/blob/main/docs/general/how-to-define-semantic-conventions.md#defining-events

Some areas to cover:

• span vs event (see also SemConv guidance: document how to define spans #1853)
• granularity / when to define a new event vs reuse the same event definition across multiple operations
• when to use event attributes vs body (Logs/Events guidance on what to put to attributes vs body #1651)
• when to use complex vs flat attributes (Semantic Conventions should provide support for code generation of AnyValue (event/complex types) fields #1748)
• when to set severity and how to do it