Clarify usage of Distributed Tracing Extension by OpenTelemetry

[joaopgrassi]

From the last messaging workgroup meeting, it was pointed out that now that we are close to merge the CloudEvents Semantic Conventions PR, we should make it clear in the CloudEvents spec that the Distributed Tracing Extension is used by OTel.

The reason for this "linking" between specs is that the extension caused confusion in the past and was even asked to be removed: cloudevents#751. Basically the idea is that OpenTelemetry "claims" this extension and by providing the semantic conventions, its usage becomes more clear.

Here is the recording of the meeting where this subject came up: https://youtu.be/gjz3Fz_hz_8?t=937

So, in this PR I intended to:

• Remove instructions and extra clarifications on how this is used - It's now in the semantic conventions
• Clear some wording (context, propagation, distributed tracing) by using the text from the OpenTelemetry specification. (For the case people aren't aware of OpenTelemetry nor heard these terms before)
• Add link to the semantic conventions document where users will find out how to use it

[pirgeo]

Looking good to me. I guess this will only be merged after the spec PR, right? the link is currently dead because the cloudevents.md is not yet merged.