[CI] Handle creation of empty Hugo-modules for deps

[chalin]

• Contributes to [CI] Docsy NPM-package: global install fails as of 0.8.0 #2122
• This is a refactoring in support of https://www.docsy.dev/docs/get-started/other-options/#docsy-npm-install-side-effect
• Introduces a bash script to replace the simple call to npx mkdirp. The script gives us more flexibility. It can be run to create empty directories for Docsy Hugo-module deps, for projects not using Hugo modules. The script accepts a path as an argument, which will make it easier to apply more generally. Also, setting DOCSY_MKDIR_HUGO_MOD_SKIP to any value will allow that postinstall action to be skipped.
• Updates the docs to explain the postinstall script behavior, and how power-users can disable it.
• Drops the dependency on the mkdirp NPM package