Document how to set projection strings for dfs2 files

[ecomodeller]

The docs don't explain how to create a correct projection/WKT string when writing dfs2 data with non-trivial coordinate systems.

Current state

• The dfs2 user guide shows "LONG/LAT" and "UTM-32" examples but doesn't explain the projection string format
• The getting-started book says "Projection string in WKT format. Use MIKE Zero for formatting" — not helpful for Python-only or Linux users
• The CRS class (mikeio.spatial.crs.CRS) already supports from_epsg() and from_pyproj() but is not mentioned anywhere in the docs

What should be documented

• The three accepted projection string formats: abbreviations ("LONG/LAT", "UTM-32", "NON-UTM"), full WKT1 strings, and conversion via CRS
• How to convert from an EPSG code: CRS.from_epsg(32632).projection_string
• The WKT1_ESRI format requirement (not WKT2) — pyproj's to_wkt(version="WKT1_ESRI")
• What "NON-UTM" means (local coordinates, no georeferencing)

Suggested location

Add a "Projections" subsection to docs/user-guide/dfs2.qmd, in the "Creating a dfs2 file" section.