Turning docsy theme into a hugo module

[deining]

This is my final approach to bringing hugo modules to docsy theme. This PR supersedes my previous attempt #520. This PR is the module part only, docu was submitted separately with PR #802.
Motivation behind my PR was already explained here. From the discussion at #535, I learned that this PR will be (hopefully!) appreciated by the Docsy community. A recent blog post about conversion from git submodule to hugo modules can serve as additional reference, I fully agree with what is stated in this blog post (citation):

Git submodules are cumbersome to manage.
In my opinion, git submodules are a sort of anti-pattern for Hugo
(I know many themes suggest using git submodules).
There’s a better way to manage your Hugo site’s theme: Hugo Modules.

Please note, that this PR is not ready for review yet, also the checks after submitting the PR will fail. That's due to the fact that inside my go.mod, I'm missing a recent commit hash that I can refer to.
There are different approaches to get this PR ready so that reviewers and/or community can try it out:

• Merge Preparations for using docsy theme as hugo module #796 into master.
• Once you are reluctant to do so, I would propose you create a new branch modules in the dosy repo, I will submit then resubmit the patch to this branch.

I hope that my PR will be well received and that it will be accepted eventually. I can't guarantee for a completely smooth transition from submodules to hugo modules, but I'm sure we can iron the wrinkles out and doing the transition is worthwile.
If you are curious, you my read my documentation submitted with PR #802.

[deining]

I just reworked and rebased my PR and IMHO it is now ready to be merged.

What I did:

config.toml:

[[module.imports]]
  path = "github.com/twbs/bootstrap"
[[module.imports.mounts]]
  source = "scss"
  target = "assets/vendor/bootstrap/scss"
[[module.imports]]
  path = "github.com/FortAwesome/Font-Awesome"
[[module.imports.mounts]]
  source = "scss"
  target = "assets/vendor/Font-Awesome/scss"

Successful deploy in Netlify, using Hugo Modules

config.toml, imports deactivated:

[[module.imports]]
  path = "github.com/twbs/bootstrap"
  disable = true
[[module.imports.mounts]]
  source = "scss"
  target = "assets/vendor/bootstrap/scss"

[[module.imports]]
  path = "github.com/FortAwesome/Font-Awesome"
  disable = true
[[module.imports.mounts]]
  source = "scss"
  target = "assets/vendor/Font-Awesome/scss"

Successful deploy in Netlify, using git submodules

So this is essentially the proof that you can switch back and forth between Hugo modules and git submodules easily, all you have to do is to set disable = true/false on two places inside config.toml.

@LisaFC: I'm very confident we can merge this PR into master without breaking anything. In my PR, I set disable = true, with this conservative setting, users won't see any difference when using the module as submodule or via git clone.

[LisaFC]

Oh this is great! I think we're safe to merge this now with the feature switched off as default and then we can look at the docs, default approach, etc. etc.