WIP: rewrite portion of Provider spec

- More sections need to be moved from "working with" to "writing a ..."
- When this is done, the whole needs to be read again and small
  adjustements are likely required to improve the flow of the doc.
- 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>