Add documentation vor v1.6.1 to tutorials

[pat-schmitt]

• Here I add a new 10 minute tutorial explaining how to use the different dynamic solvers (semi-implicit and flux-based) and as an extra explaining different bed-shapes for the downstream line (Closes Tutorial for SemiImplicitModel oggm#1515)
• I added a short explanation about the new flowline diagnostics flux_divergance, dhdt and climatic mass-balance (Closes Add new flowline diagnostics to tutorials oggm#1597)
• I added the new dynamic spinup functionalities (target_yr and target_value) to the tutorials (Closes Add new dynamic spinup functionality to tutorials oggm#1601)
• I added a short explanation about the possibility to keep multi polygon outlines in oggm (related to Add possibility of using MultiPolygon Outlines for Elevation Band Flowline oggm#1604)

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[fmaussion]

Fantastic thanks @pat-schmitt !