Update Documentation to Simplify Authentication

[danielbankhead]

Currently, in numerous places we suggest awaiting auth.getClient() rather than directly passing GoogleAuth:

• https://googleapis.dev/nodejs/googleapis/latest/sheets/
• https://github.com/googleapis/google-api-nodejs-client/blob/main/samples/jwt.js

GoogleAuth has been supported as an auth param for a while now:

• feat: support GoogleAuth parameter google-api-nodejs-client#2210

We should update our documentation to simplify and reduce complexity.

Ref: #779