docs: audit and update all guides

[ocots]

Summary

Comprehensive documentation audit and update for CTModels.jl.

New guide

• docs/src/model/display.md: Documents the CTModels.Display module — Base.show extensions for Model and PreModel, mathematical rendering, abstract definitions, and the RecipesBase.plot stub.

Audited guides (16 files updated)

Model guides:

• model/overview.md: Added Display guide to reading order table.
• model/types_and_traits.md: Added ConstraintsModel to the noun-family table; added is_initial_time_fixed/is_initial_time_free/is_final_time_fixed/is_final_time_free predicate accessors.
• model/components.md: Added initial_time accessor mention; added "Symbolic definitions" section covering Definition, EmptyDefinition, and expression.
• model/constraints.md: Added raw constraint accessors (path_constraints_nl, boundary_constraints_nl, state/control/variable_constraints_box); added isempty_constraints and constraint references.
• model/building.md: Added has_abstract_definition, is_abstractly_defined, isempty_constraints to the accessor example; documented get_build_examodel.

Solution guides:

• solution/overview.md: Added is_empty, is_empty_time_grid, has_duals, model to the anatomy table.
• solution/time_grids.md: Added "Checking for emptiness" section with is_empty and is_empty_time_grid.
• solution/duals.md: Added boundary_constraints_dual accessor; added box-constraint dual accessors table (lb/ub for state/control/variable); added dim_dual_* accessors; added has_duals/EmptyDualModel section; added model accessor.
• solution/trajectories.md: No changes needed (already complete).

Initial guess guides:

• initial_guess/overview.md: Added AbstractInitialGuess/AbstractPreInitialGuess type hierarchy mention.
• initial_guess/formats.md: Added "Per-component constructors" section documenting initial_state, initial_control, initial_variable.
• initial_guess/validation.md: No changes needed (already complete).

Serialization guides:

• serialization/overview.md: No changes needed (already complete).
• serialization/export_import.md: Improved low-level tags note with proper @ref links to JLD2Tag, JSON3Tag, AbstractTag.
• serialization/plotting.md: No changes needed (already complete).

Navigation updates

• docs/make.jl: Added "Displaying Models" => "model/display.md" to the OCP Model section.
• docs/src/index.md: Added Display guide to the developer guides list.
• docs/src/getting-started.md: Added Display guide row to the Next Steps table.

Build verification

• julia --project=docs docs/make.jl builds cleanly with no cross-reference warnings.