docs: extract sections in how to write integration tests to their own howto guide

[tromai]

Description

This PR made changes to the How to write integration tests for a charm page.

We want to keep this page generic, and move some topic specific examples to their own how-to guides.

• Actions
• Relations
• Configuration
• Deploying with resources

TODO

• Look for real world examples of these topics and add link (for example, at the end of this section).
  • Make sure the examples are not too complicated, and write up some description for each of them. I decided to use a relative link, and don't point to the exact line.
• Try the examples in those pages in a sample charm.