WIP: Use Hugo modules

[LorenzBischof]

The following PRs have added Hugo Modules support to Docsy:
google/docsy#871
google/docsy#802

I would wait until v0.2.0 is released.

To use the the docsy-plus module the following can be added into the config:

theme = ["github.com/acend/docsy-plus"]

It might even make sense to have docsy-puzzle and docsy-acend manage the docsy-plus theme.

See https://gohugo.io/hugo-modules/use-modules/ for more information on how to use Hugo Modules.

I had some problems with Module versions. The required module versions from https://github.com/google/docsy/blob/master/go.mod must be added to go.mod within this repository. Running hugo mod tidy should add these automatically.

[splattner]

is this PR still needed? @phil-pona

[LorenzBischof]

It depends if you would like to use Hugo Modules instead of Git Submodules. Docsy now recommends using Hugo Modules, but still supports Git Submodules.

This was left WIP because the Docsy version supporting Hugo Modules was not released yet. Since the README now contains references to Hugo Modules, I guess you could use it now.

[phil-pona]

The PR needs to be adjusted though, the font and boostrap deps are part of the github.com/google/docsy/dependencies
Currently all Trainings Repos use Go Hugo modules instead of Git Submodules already.

I'm going to try to implement this one today. After that we can finally rearrange the deps so that the docsy and docsy dependencies are deps from the docsy plus instead of the actual Training module, which will make updating those dependencies a lot easier and more straightforward

[phil-pona]

implemented with #13