Eliminate Bootstrap and Font Awesome git submodules

[chalin]

• Followup to Use NPM packages for Bootstrap and Font Awesome #870:

  As discussed during the 2022-01-31 PSC meeting, we'l be exploring whether we can eliminate the git submodules to Bootstrap and Font Awesome. ...

Tasks:

• Drop Bootstrap and Font Awesome git submodules #889
• Also clean out the User Guide package scripts (used to remove BS & FA from assets/vendor/* before building) - Remove obsolete git-submodule scripts from UG package.json #1073
• Update the docs & provide migration information - Docsy 0.4.0 migration instructions and doc updates #1074

[chalin]

PR #889 implements this, and you can see that the site builds just fine:

• Build log, see (for example): https://app.netlify.com/sites/docsydocs/deploys/6242018db28d930008d866b9
• Preview, see https://deploy-preview-889--docsydocs.netlify.app/

[chalin]

@geriom, et all. Some form of the following question was asked during one of our recent PSC meetings: what harm can result from keeping the git submodules even though they are no longer needed by Docsy.

I think that #755 (comment) offers one concrete example. Though the issue mentioned is specifically w.r.t. depth, it wouldn't be an issue if the submodules weren't there :).