[BUG][docs] Access Docsy as a Hugo module or Git submodule, not both

[chalin]

This website is currently linked to a 5-year old version of Docsy via Git submodules. The following PR switched to Hugo modules:

• Move into hugo modules and adapt existing code and docs #131

But then a followup PR brought back Docsy via git submodules:

• Sync documentation from shipwright-io/build at v0.14.0 level #140

If you've decided to use Hugo modules, then the submodule should be cleaned out once again.