Point types documentation and FDD calculation

[claudiaalvgar]

A new manual page is added to document the PointTypes behaviour. The six bit flags, their combined values, how they are used by is_depleted, get_active_volume, and estimate_depletion_voltage to exclude the inactive layer, how mark_inactivelayer_bits! propagates the inactive layer flag outward from the doped contact, and a detailed description of how the new get_FDD function extracts the two contours and computes the FDD thickness.

Example plots show the inner contour overlaid on the point-type map and a heat-map of FDD thickness across the detector cross-section.

The new function get_FDD, computes the Full Depletion Depth (FDD), the thickness of the inactive lithium-doped layer. For each inner contour point it returns the coordinates of the transition between the inactive layer and the active bulk (r_inner/z_inner), the coordinates of the nearest point on the doped contact surface (r_outer/z_outer), and the perpendicular thickness of the inactive layer.
When the impurity model is PtypePNJunctionImpurityDensity or ThermalDiffusionLithiumDensity, the thickness is computed analytically via ConstructiveSolidGeometry.distance_to_surface, giving exact perpendicular distances for any surface orientation (vertical, horizontal, or diagonal). For other models it falls back to the Euclidean distance to the nearest contact-surface grid tick. Both Cylindrical and Cartesian variants are implemented.

The outer contour (r_outer/z_outer) is collected from grid ticks that carry inactive_contact_bit directly, which lie on the contact surface itself since SSD places grid ticks at contact boundaries. This means that for axis-aligned surfaces the Euclidean distance between inner and outer contour points equals the analytical thickness exactly; for diagonal surfaces (e.g. a cone face) a small residual difference remains because the nearest contact tick is not the exact perpendicular foot.
A helper function _dist_to_inactive_contact is added to extract the distance_to_contact from the impurity density model, or return nothing to trigger the Euclidean fallback. Tests for get_FDD are added to test_depletion.jl