docs: add a simple example of instantiating each client at the top of each service client and in the README

[bshaffer]

For the reference documentation, it would be great to include a simple example of instantiating a client

for instance, when a user lands on the landing page for VertexAI, we could have an example of a simple call (pulled from the samples/ directory, for instance). Similary, at the top of each Service, we could include a similar example of creating the client.