docs(domains): tone continuum abstract docstrings for removed files

Strip decorative markdown emphasis, drop Kirchhoff plate pointer, and remove
See-Also links to deleted formulations.jl / missing design doc paths.

src/domains/continuum/abstract.jl

@@ -17,27 +17,27 @@ Concrete types are in types.jl, implementations are in theory-specific files.
 
 Abstract type for discretization formulations.
 
-**DESIGN PHILOSOPHY: Formulations are DOMAIN-AGNOSTIC dimensionality concepts.**
+DESIGN PHILOSOPHY: Formulations are DOMAIN-AGNOSTIC dimensionality concepts.
 
 # Key Distinction: Formulation vs Theory
 
-**Formulation** (domain-agnostic):
+Formulation (domain-agnostic):
 - Describes DIMENSIONALITY and geometric simplifications
 - Used by multiple physics domains
 - Examples: FullThreeD, Axisymmetric
 - Can be reused across continuum, heat, acoustics, etc.
 
-**Theory** (domain-specific):
+Theory (domain-specific):
 - Describes PHYSICS assumptions (stress/strain, kinematics)
 - Only meaningful for one physics domain
 - Examples: PlaneStress (continuum), Kirchhoff (plates)
 
 # Why Separate Them?
 
-**Problem**: Heat transfer needs FullThreeD and Axisymmetric, just like continuum!
+Problem: Heat transfer needs FullThreeD and Axisymmetric, just like continuum!
 If FullThreeD is defined in domains/continuum/, heat can't use it without duplication.
 
-**Solution**: Formulations are dimensionality (shared), theories are physics (domain-specific).
+Solution: Formulations are dimensionality (shared), theories are physics (domain-specific).
 
 # Examples
 
@@ -49,13 +49,10 @@ Axisymmetric()            # Used by: continuum, heat, etc.
 # Domain-specific theories (in domains/*/types.jl)
 PlaneStress               # domains/continuum/types.jl
 PlaneStrain               # domains/continuum/types.jl
-Kirchhoff                 # domains/plates/types.jl
 ```
 
 # See Also
 - Concrete formulation types: `continuum/types.jl` (ContinuumFormulation)
-- Formulation implementations: `continuum/formulations.jl`
-- Architecture docs: `docs/src/design/formulations_and_theories.md`
 """
 abstract type AbstractFormulation end
 
@@ -64,43 +61,42 @@ abstract type AbstractFormulation end
 
 Abstract type for continuum mechanics theories.
 
-**Domain-specific physics assumptions** for solid mechanics.
+Domain-specific physics assumptions for solid mechanics.
 
 # Theories
 
-**PlaneStress (σ_xx, σ_yy, σ_xy, σ_zz = 0):**
+PlaneStress (σ_xx, σ_yy, σ_xy, σ_zz = 0):
 - Thin plates and membranes (thickness << length/width)
 - Out-of-plane stress σ_zz = 0
 - Examples: Sheet metal, aircraft skin, thin-walled structures
 
-**PlaneStrain (ε_xx, ε_yy, ε_xy, ε_zz = 0):**
+PlaneStrain (ε_xx, ε_yy, ε_xy, ε_zz = 0):
 - Thick sections with no variation in z-direction
 - Out-of-plane strain ε_zz = 0
 - Examples: Dams, tunnels, retaining walls, long cylinders
 
-**FullThreeD:**
+FullThreeD:
 - No simplifications, all six stress/strain components
 - Most accurate but most expensive
 
-**Axisymmetric:**
+Axisymmetric:
 - Geometry and loading symmetric about z-axis
 - No circumferential variations (∂/∂θ = 0)
 - Examples: Pressure vessels, pipes, rotating disks
 
 # Mathematical Details
 
-**Plane Stress (thin plate):**
+Plane Stress (thin plate):
 - Stress state: σ_zz = σ_xz = σ_yz = 0
 - Strain: ε_zz ≠ 0 (computed from σ_zz = 0 condition)
 - Constitutive: 3×3 reduced stiffness matrix
 
-**Plane Strain (thick section):**
+Plane Strain (thick section):
 - Strain state: ε_zz = γ_xz = γ_yz = 0
 - Stress: σ_zz ≠ 0 (computed from ε_zz = 0 condition)
 - Constitutive: 3×3 reduced stiffness matrix (different from plane stress!)
 
 # See Also
 - Concrete theories: `continuum/types.jl` (FullThreeD, PlaneStress, PlaneStrain, Axisymmetric)
-- Theory implementations: `continuum/formulations.jl`
 """
 abstract type AbstractContinuumTheory end