Documentation: (for maintainers) how to update the version of the frontend pacakge

[marlo-longley]

Descriptive summary

Document the changes made here: geoblacklight/geoblacklight#1433. See PR review discussion.
Maintainers need to know that the frontend package version is not in package.json but rather in lib/generators/geoblacklight/install_generator.rb

Find a place in geoblacklight.org/docs for this info.