docs: add details about DI mechanism, esp. default providers

[paullatzelsperger]

What this PR changes/adds

Adds more details about the dependency injection mechanism, in particular about default provider methods.

Why it does that

Default provider methods have been a repeated source for problems due to them being misused. This document should avoid that.

Further notes

• at some point we'll have to go through the code base and correct usages of default provider methods. A subsequent PR will do that.

Linked Issue(s)

Closes #1806

Checklist

• added appropriate tests?
• performed checkstyle check locally?
• added/updated copyright headers?
• documented public classes/methods?
• added/updated relevant documentation?
• assigned appropriate label? (exclude from changelog with label no-changelog)
• formatted title correctly? (take a look at the CONTRIBUTING and styleguide for details)