Add details of review process to Contributor Handbook

[spier]

We have used the new maturity levels for the patterns for over a year now.
I figured we could aggregate our working knowledge in the Contributor Handbook, to make it easier for new contributors to understand what we are after when using the different maturity levels.

This PR also starts to implement #170 and #171.

Changes made:

• Adds information about automatic checks
• Describe some unspoken review expectations
• Adds where the output artifacts in each level are published
• Document where to keep assets (images etc) that are used in patterns.
  • Most are kept in /assets, while the 3 project-setup patterns keep the assets in their own sub-folders
• (basic) first Style Guide and Glossary

[spier]

@maxcapraro as you wrote the first version of the Contributor Handbook, can you help us understand the intended semantic difference between the Validation requirements and Content requirements?

Understanding this better will help to make better decisions about which info to put where.

[spier]

@tapjdey @WillemJiang as you have made your first contributes to our Patterns recently, it would be great to get your feedback here.

This PR tries to explain better how we go about reviewing patterns.

You can focus your feedback on levels 1 and 2, as we don't have level 3 patterns yet.

If you have any further thoughts about this, let us know. And thanks for you help!

[spier]

@robtuley as you recently worked on a pattern, I was wondering if you could review if this PR here is a helpful improvement for the documentation in our Contributor Handbook?

[robtuley]

This looks like a helpful update to me as a recent contributor.