README simplification of data plane libraries

Refer to: https://github.com/JonathanGiles/azure-sdk-readme-refresh. We'll need to determine which libraries can be considered "tier 1" for this endeavor. Other tiers can follow suit at a later point throughout the year and, after some point, we will also probably need other languages to align with this way of doing documentation.

We should additionally check if other documentation online (e.g. MSDocs) is up to date and points to the right places.