…5637)

# Description

Restructures the physics-backend documentation to give each backend a
first-class home and to add the cross-backend orientation pages asked
for in
[isaac-sim/IsaacLab-Internal#876](isaac-sim/IsaacLab-Internal#876).

**Structure**

```
docs/source/overview/core-concepts/physical-backends/
├── index.rst                 ← user-facing hub + feature-support matrix
├── solver-comparison.rst     ← cross-backend behavioural differences
├── physx/
│   ├── index.rst
│   ├── installation.rst
│   ├── configuration.rst     ← PhysxCfg tuning knobs
│   └── supported-features.rst
├── newton/
│   ├── index.rst
│   ├── installation.rst
│   ├── supported-features.rst   (was experimental-features/.../limitations-and-known-bugs.rst)
│   ├── mjwarp-solver.rst        (was experimental-features/.../solver-transitioning.rst)
│   └── kamino-solver.rst        (was experimental-features/.../using-kamino.rst)
└── ovphysx/
    └── index.rst             ← stub flagged as highly experimental; tracking issue #5634
```

**Highlights**

- The Newton subdir moves wholesale out of `experimental-features/` via
`git mv`; framing changes from "experimental feature branch" to "beta
backend." The Experimental Features toctree now contains only
`bleeding-edge`.
- Per-solver Newton pages (`mjwarp-solver`, `kamino-solver`) replace the
old `solver-transitioning` / `using-kamino` files, matching #876's
"sub-sections for the Newton solvers" ask.
- New PhysX page set written from `isaaclab_physx.physics.PhysxCfg`,
mirroring the Newton structure.
- OvPhysX stub references the full in-flight PR set (#5421, #5422,
#5426, #5459, #5570, #5589) and is gated by a follow-up issue (#5634)
for expansion after those land.
- `solver-comparison.rst` covers friction, contact pipeline,
restitution, stabilization, convergence, articulation coordinates,
substepping, and GPU buffers across PhysX TGS, Newton MJWarp, and Newton
Kamino. Each cell points at the concrete config attribute that controls
the behavior, with a porting checklist at the end.
- Newton's `supported-features.rst` list refreshed against `develop`'s
actual `newton_mjwarp` coverage (adds Shadow Hand, Shadow Hand Over,
cabinet, dexsuite, rough-terrain locomotion). Replaces the bit-rotting
bullet list with a discovery recipe (`grep -rln newton_mjwarp
source/isaaclab_tasks/`).
- Cross-references updated in `docs/index.rst`,
`core-concepts/index.rst`, `multi_backend_architecture.rst`,
`features/visualization.rst`, and
`overview/reinforcement-learning/rl_existing_scripts.rst`.

Fixes isaac-sim/IsaacLab-Internal#876

## Type of change

- Documentation update

## Screenshots

N/A — docs-only restructure. New page tree visible from the
table-of-contents in `core-concepts/`.

## Checklist

- [x] I have read and understood the [contribution
guidelines](https://isaac-sim.github.io/IsaacLab/main/source/refs/contributing.html)
- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./isaaclab.sh --format`
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works *(N/A — docs-only)*
- [ ] I have added a changelog fragment under
`source/<pkg>/changelog.d/` for every touched package *(N/A — docs-only;
matches precedent of #5512 and 4aeb4d6 which shipped without
fragments)*
- [x] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there