User guide config: use mounts rather than the old contentDir param

[chalin]

• Closes Config: clean up language settings #897
• There are no changes to the generated site files:

  $ npm run build && gb
  ...
  $ (cd public && git diff)
  $

/cc @LisaFC @geriom @emckean

[chalin]

@deining - can you confirm that this doesn't break anything for the Hugo modules support? If you don't have time, no worries, I'll look into it next week. Thanks!

[deining]

@deining - can you confirm that this doesn't break anything for the Hugo modules support?

Fine for me: after applying your changes, I still could run successfully the userguide using docsy as hugo module.

[chalin]

Thanks for checking @deining!