Document the differences between distros and extensions

[trask]

Motivated by #6041

Trying to come up with a list of things relevant to users (not vendors) that differentiate these two approaches.

Extension pros:

• Much simpler to build (e.g. doesn't require package relocations)
• Don't need to rebuild with each OpenTelemetry Javaagent release (once extension API is declared stable at least)

Distro pros:

• Can add your own dependencies to the bootstrap class loader

[trask]

Closing, can revisit later if anyone has renewed interest.