[Docs] Expand OvPhysX backend page when in-flight integration PRs land

[AntoineRichard]

The new per-backend documentation hub added under
docs/source/overview/core-concepts/backends/ (see #876) ships a stub
OvPhysX page flagged as highly experimental:
docs/source/overview/core-concepts/backends/ovphysx/index.rst.

Once the in-flight OvPhysX integration PRs land, this stub should be
expanded into a full page mirroring the Newton and PhysX structures
(intro, installation, configuration, supported-features).

Tracking PRs

• Articulation: [OVPHYSX] Articulation rewrite (data class + asset class + kernels) #5459
• RigidObject: [OVPHYSX] RigidObject + RigidObjectData asset #5426

Acceptance

• Replace the stub ovphysx/index.rst with a full page set, or convert
  it to a multi-page subdir (ovphysx/installation.rst,
  ovphysx/configuration.rst, ovphysx/supported-features.rst).
• Document the kit-less invocation flow and the USD PhysicsScene →
  scene-level config mapping.
• Update the feature-support matrix in backends/index.rst to drop
  "Partial — see backend page" rows where OvPhysX coverage is now
  complete.