Document schema fragment framework in API reference

[vidurv-nvidia]

Description

The schema-cfg "fragment" refactor introduced a new public API in isaaclab.sim.schemas — single-namespace fragment classes and family writers — but the rendered API reference still listed only the legacy define_*/modify_* writers and *PropertiesCfg classes. This documents the fragment API that is already merged into develop.

Added to docs/source/api/lab/isaaclab.sim.schemas.rst:

• Schema fragments rubric + detail section: SchemaFragment, the per-family marker bases (RigidBodyFragment, CollisionFragment, MassFragment, MeshCollisionFragment, FixedTendonFragment, SpatialTendonFragment), and the solver-common UsdPhysics*Cfg fragments (UsdPhysicsRigidBodyCfg, UsdPhysicsCollisionCfg, UsdPhysicsMeshCollisionCfg).
• The family writers: apply_namespaced, apply_rigid_body_properties, apply_collision_properties, apply_mass_properties, apply_mesh_collision, apply_mesh_collision_properties, apply_fixed_tendon_properties, apply_spatial_tendon_properties — wired into their respective sections.

Scope is the API reference only; the narrative concept page (schema_cfgs.rst) rewrite is deliberately out of scope. Articulation-root and joint-drive fragment symbols are documented in their own PRs (#6272, #6273), since those symbols are not yet in develop.

Type of change

• This change requires a documentation update

Checklist

• I have run the pre-commit checks with ./isaaclab.sh --format
• I have added a changelog fragment (docs-only .skip)
• I have updated the changelog accordingly (handled by CI)

[vidurv-nvidia]

Closing as superseded: the schema-fragment framework documentation now lands directly with the feature work — the core concepts page and API reference in #6640, with the deformable families section added in #6673. No unique content remains here.

[vidurv-nvidia]

Reopening — closed by mistake. Still superseded by #6640/#6673 but that's a separate decision.