Auto-sync vendor assets & Hugo module versions + copyedits to README and CONTRIBUTING

[chalin]

• Followup to Offer Docsy as a single Hugo Module #1217
• Fixes Ensure that versions of dependencies remain in sync #1182
• Fixes Automate the updating of vendor assets #1723
• Creates a single assets/_vendor folder to hold all missing vendor assets (which for now only contains the bootstrap vendor file)
• Adds a script to sync the BS vendor file(s) from the BS NPM package, triggered by a post-install event
• Adds a script to sync Hugo modules based on the corresponding NPM package versions. Hence the NPM version are canonical
• Updates CONTRIBUTING.md:
  • Simplifies the steps in Publishing a release since some changes are not handled automatically. Also removed references to the dependencies module that was dropped via Offer Docsy as a single Hugo Module #1217
  • Other tweaks
• Explicit mention of code-of-conduct in README and CONTRIBUTING.md

[chalin]

Closing in favor of #1725