Improve examples and code documentation of IDE-SECIR model

[annawendler]

Motivation / Current Behaviour

• The examples ide_secir and ide_secir_ageres have a lot of code duplication. Also, the age-resolved example doesn't work for arbitrary numbers of age groups.
• There has been some feedback on where the documentation of the implementation could be improved.

Enhancement description

• Merge examples ide_secir and ide_secir_ageres
• Add documentation

Additional context

No response

Checklist

• Attached labels, especially loc:: or model:: labels.
• Linked to project