Documentation for adapter refactoring

[bossenti]

Body

In #1289 we introduced a large refactoring for our adapters. This significantly improves the way we can define adapters. Therefore, we should write and adapt the existing documentation to answer the following questions:

• [x] how to create an adapter with the new interfaces?
• [ ] how to implement additional parsers?
• [ ] how to use the adapter migration?

StreamPipes Committer

I acknowledge that I am a maintainer/committer of the Apache StreamPipes project.

[bossenti]

The first question is taken by https://github.com/apache/streampipes-website/pull/124