Fixes open-telemetry#1656

The documents that describe transformations to Jaeger and Zipkin contained
duplicate information about how to record InstrumentationLibrary.

They also contained incomplete and incorrect information about how to record
dropped attribute count.

This change extracts common transformation rules into a separate document (non-otlp.md)
and references this document where appropriate.

We also define otel.event.dropped_attributes_count as the only way to record
dropped attribute count, regardless of the entity for which it is recorded.
Previously in Jaeger format we specified that for Events we must use
otel.event.dropped_attributes_count and did not specify at all what to use for Spans.

I believe using the same uniform key for Spans and Events is correct.

I consider this a specification bug and do not believe it should be treated as
a breaking change.

Also delete the Zipkin mapping for LocalChildSpanCount since there is no such
OpenTelemetry field anymore.