Document restrictions for extension installation

[fmoor]

Currently the help for gel extension install doesn't mention that it can only install extensions for local development servers. This should be mentioned in the help text along with a reference to instructions for installing extensions in other contexts.

$ gel extension install --help
Install an extension

Usage: gel extension install [OPTIONS] <EXTENSION>

Arguments:
  <EXTENSION>  Name of the extension to install

Options:
  -I, --instance <INSTANCE>  Name of the instance
  -h, --help                 Print help

$ gel --version
Gel CLI 6.2.0-dev.1267+609b673