add 'mage alias' fix help w/o bb installed

[escherize]

mage alias

• Prints instructions to install mage alias + autocomplete for bash, zsh, and fish shells.

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When bb is not installed globally the print-help function was failing because it runs bb from /path/to/metabase not /path/to/metabase/bin! Now it calls the auto-installed babashka executable instead.

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Updates comments for the example task with better guidance + easier to understand