docs(automodel): rename the DTensor backend to Automodel

[yuki-97]

Part of the DTensor v1 removal series -- overall status is tracked in the description of #4198.

What does this PR do ?

Renames the DTensor backend to Automodel across the docs. 42 files. Part 1/3 of the Automodel
rename; the code and the dtensor_cfg key follow in their own PRs.

What changed

1. Wording, 79 replacements in 32 files. Since v1 was removed there is one non-Megatron backend
and its name is Automodel, so the label spellings it accumulated all collapse: DTensor,
DTensor (FSDP2), AutoModel (DTensor), Native PyTorch (DTensor), and docs/about/backends.md's
PyTorch.

2. fsdp2-parallel-plan.md -> automodel-parallel-plan.md, and its Fallback Priority section
rewritten: it described three tiers from the deleted parallelize.py, where Automodel actually has
four.

3. dtensor-tp-accuracy.md -> automodel-tp-accuracy.md, with its assets. The published URL is
hardcoded in two places outside docs/ -- nemo_rl/models/policy/lm_policy.py (a runtime error
message the user reads) and tests/unit/test_config_validation.py -- so those move with it rather
than being left pointing at a 404.

Both renames break the old published URLs; the repo has no redirect mechanism in docs/conf.py.

Deliberately not changed

1. DTensorPolicyWorkerV2 (nsys-profiling.md, automodel-tp-accuracy.md) -- part 2/3 renames
the class.

2. policy.dtensor_cfg (45 references) -- it is still the real key name. Part 3/3 renames it to
automodel_cfg.

3. Recipe filenames carrying fsdp2 / dtensor, which the docs link to -- being handled by the
test-suite series, #3557.

4. DTensor as the PyTorch tensor type -- DTensorRef, XferDTensor fraction, "DTensor-style
Shard/Replicate placements", the CP mermaid diagrams. These describe the mechanism, not the backend,
and stay.

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