[UG] Give guidance on what to do on "failed to load modules" error

[chalin]

• Fixes [CI] Docsy 0.8.0 NPM-package use may fail #2116
• Gives guidance on what to do if Hugo reports a "failed to load modules" error when using Docsy via NPM
• Ran Prettifier over the updated files
• Makes setting of themesDir in the sample config the "happy path"
• Preview: https://deploy-preview-2117--docsydocs.netlify.app/docs/get-started/other-options/#option-3-docsy-as-an-npm-package

[chalin]

/cc @sftim

[sftim]

A related concern; the postinstall I think runs cp -R node_modules/bootstrap/scss/vendor/* assets/_vendor/bootstrap/scss/ and this is making assumptions about both the current working directory and the name of the install target.

Seems like NPM might not guarantee those are always the right values to make that command work.

[chalin]

For the cp command, we've switched to use of the NPM cpy-cli command. As for the working directory, I'll leave that as is for now. We can address any specific issues if/when they arise.

[chalin]

@sftim - btw, unless you run ci:prepare, you won't hit the copy commands. The ci:prepare script is only for those maintaining and releasing Docsy.