Make the pg_tde README great again - Rewrite the first part and default installation method

It’s essential for users to be easily able to install and setup any tool. Therefore, the provided documentation needs to help and to guide them in the easiest way possible. Currently, with the change of tde_heap_basic vs tde_heap our README is hard to follow.

My proposal would be to rewrite the beginning of the README, to clearly mention the following:

• We do have two access method tde_heap_basic and tde_heap

• What are the functional differences and how do their differ from the architectural design?

• How do I install tde_heap_basic via packages?

• How do I install tde_heap via packages?

• …After that we can talk about “building from source” or “docker” etc. but keep the start crisp and focused.