Resolve "Document model solver loop and time step adaptation schemes in user manual"

What does this MR do?

Add a new user manual page for documenting the time step schemes of the models in DORiE.

In contrast to the draft in #174 (closed), I decided to go for regular enumerations to depict the algorithms, rather than using diagrams.

Is there something that needs to be double checked?

No.

Can this MR be accepted?

• Added documentation on time step schemes
• Pipeline passing
• Squash option set
• Delete branch option set
• Added entry to CHANGELOG.md

Assignee: If the Squash option is set, check/update the commit message right before merging!

Related issues

Closes #174 (closed)