Convert project to use docsy as hugo module

[deining]

This PR

• converts the project to make use of docsy as hugo module
• updates docsy submodule to latest version prior to update to bootstrap 5
• adds a section to README.md, explaining how to use the repo as template for a new site
• netlify.toml: brings hugo lo latest version 0.110.0

[deining]

From the netlify deploy log:

10:18:54 PM: ---------------------------------------------------------------------
  UNSUPPORTED BUILD IMAGE

  The build image for this site uses Ubuntu 16.04 Xenial Xerus, which is no longer supported.

  To enable builds for this site, select a later build image at the following link:
  https://app.netlify.com/sites/mostlydocs/settings/deploys#build-image-selection

  For more details, visit the build migration guide:
  https://answers.netlify.com/t/please-read-end-of-support-for-xenial-build-image-everything-you-need-to-know/68239
  ---------------------------------------------------------------------

@gwatts: can you bring the build image to a newer version, please? Thanks!

[gwatts]

@deining this has been on my backlog to do for a long time; thanks for taking it on! Will review and merge asap

[deining]

@deining this has been on my backlog to do for a long time

Any plany to review (and eventually merge) this PR?
If so, could you please bring the Netlify build image to a newer version, please? Once this is done, I will bring my PR up-to-date afterwards.

Background: Recently, I started a new docsy based documentation project and was reminded on my PR in this context.