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

[chalin]

• Supersedes Auto-sync vendor assets & Hugo module versions + copyedits to README and CONTRIBUTING #1725
• 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
• Closes Auto-sync vendor assets & Hugo module versions + copyedits to README and CONTRIBUTING #1724 by superseding it (I needed to commit to the docsy repo so that I could test from the PR branch)

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The only change in generated site files from before this PR and after:

[chalin]

@deining wrote #1725 (review):

Spotted a typo which needs to be corrected. LGTM otherwise, haven't tested it, though.

Typo fixed. Build is green. Merging.

[chalin]

Ok, good, main is back in a sane state: