docs: Rewrite portion of Provider spec

Plus addressing of a couple of nits.

The following must still be picked up at a later moment:

- More sections need to be moved from "working with" to "writing a ..."
- Documentation flow can likely be improved once the sections have been
  moved.
- Technical description of the behavior of the Provider types could be
  improved, this should be easier to do when everything has the same
  format.

Signed-off-by: Hidde Beydals <hello@hidde.co>