Introduce the ability to define namespaces #802

[thompson-tomo]

Closes: #802

With the introduction of namespaces to the semconv schema we gain the ability to define in Yaml the details of the namespace needed to generate the documentation.

This would help to enable open-telemetry/semantic-conventions#2327 to be progressed as now that general information about the namespaces can be placed on the namespace and used for documentation. At the same time we are progressing towards auto generated migration documents

This also further supports the introduction of the event registry as currently some of the event pages contains meta data about the topic which would now be able to reside as the note for the namespace.

[jsuereth]

I believe this change, and the PRs in semantic conventions likley need a "design" phase where you can outline the end goal and design.

For changes of this magnitude I'd recommend trying to have a more design focused PR (e.g. #348), or a design in a bug where we can evaluate trade-offs and help us see the forest for the trees.

[thompson-tomo]

@jsuereth sorry for the delay, i have converted this to a draft and have finally sat down and prepared #867 which talks about some of the use cases for being able to define namespaces.