[Doc] Document CatFrames with collectors and image replay buffers

[YeonwooSung]

Description

Adds a focused Sphinx recipe for using CatFrames with a data collector and a replay buffer on images, which was missing from the reference docs.

Two legitimate placements are documented and cross-linked:

• Env-side (docs/source/reference/envs_transforms.rst): stateful stacking for the policy, reset / InitTracker behavior, and dim=-3 for CHW images.
• Buffer-side (docs/source/reference/data_replaybuffers.rst): store unstacked raw pixels, rebuild the stack in rb.sample(), and wire a Collector through extend / sample. Shows both CatFrames.make_rb_transform_and_sampler and the explicit SliceSampler form.

The recipe also covers why raw uint8 frames are what you store, how the collector's ("collector", "traj_ids") interact with the slice sampler, and the common pitfalls of stacking twice (env and buffer on the same key) or using the vector default dim=-1 on pixels.

Copy-paste snippets use # doctest: +SKIP where gym / pixel rendering is required. The CatFrames class docstring now points at both sections.

Motivation and Context

Using CatFrames for inference was already documented; reconstructing a frame stack when sampling from a replay buffer was not, especially for images (stack dim -3, not the vector default). Visual RL relies on this pattern, and the collector + extend + sample path is easy to get wrong (double stacking, wrong dim, no time axis).

close #2618

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[pytorch-bot]

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❌ 4 New Failures, 3 Cancelled Jobs

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• Lint / python-source-and-configs / linux-job (gh)
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• Unit-tests on Linux / tests-cpu (3.10, mp) / linux-job (gh)
  test/test_loggers.py::TestMLFlowLogger::test_log_hparams
• Unit-tests on Linux / tests-stable-gpu (3.12, 13.0, 3) / linux-job (gh)
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• Libs Tests on Linux / unittests-gym (3.10, 12.8) / linux-job (gh)
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