NPM dependencies: fix dep install timing issue

[chalin]

• Followup to:
  • Offer Docsy as a single Hugo Module #1217
  • Auto-sync vendor assets & Hugo module versions + copyedits to README and CONTRIBUTING #1733
  • The latter broke Docsy NPM installs
• Fixes install timing issue of Docsy dependencies: we can't copy over the bootstrap (BS) vendor files from the BS NPM package until it is installed 🤷🏼‍♂️. This PR delegates the copying of vendor files to the prepare step.