Improve documentation to push for using pet search

[RamiAwar]

Pet exec has some downsides:

• Command is not stored in history
• Some commands don't execute as expected due to the fact that they run in a subshell (#112 and others)
• Does not allow editing the command before executing it

The suggestion here is to push users away from using pet exec and instead leaning on pet search with the shell shortcuts we've provided.

This would solve all of the above issues and more that I can't think of right now.