Update the doc content creation instructions

[dwelsch-esi]

Make changes to improve the effectiveness of the KEDA documentation contributor instructions.

Use-Case

Contributors, especially documentation contributors, might have trouble finding the information they need to get started.

Specification

Make the following changes to the keda-docs README:

• Move the "Become a listed KEDA user!" and "Become a listed KEDA commercial offering!" sections into their own files, or move them to the bottom of the README, so they're not at the top of the keda-docs README.
• Combine "Adding scaler documentation" and "Writing documentation for a scaler" so that they're not separated by "Writing documentation for a new authentication provider".