[9.x] Update Sail shell alias docs

[jessarcher]

This PR updates the "Configuring a Bash Alias" section of the documentation by making the following changes:

• Renames the section to "Configuring A Shell Alias" because this is not Bash-specific, and Bash is no longer the default shell on macOS.
• Updates the alias to not depend on bash. sh should be a safe alternative, but it also seems to work fine without specifying a shell.
• Adds a note on adding this to the user's shell configuration to make it always available. I've seen people run the alias command in their terminal, run sail up, and then open a new terminal and are understandably surprised when sail artisan migrate doesn't work. I'm not sure if specifying actual filenames is a little dicey, but hopefully, it at least helps the user understand that they need to take further steps to make the alias "stick".

I'd love for someone on macOS and Windows to confirm whether these changes make sense in those environments!