[doc-only] docs: drop cuda.coop, removed upstream by CCCL

[aryanputta]

Closes #2429.

Summary

Deletes the cuda.coop entry from the top-level component list. CCCL removed
cuda.coop, so https://nvidia.github.io/cccl/unstable/python/coop.html returns
404 and cannot be repointed:

• NVIDIA/cccl python/cuda_cccl/cuda/ now contains only cccl and compute
• NVIDIA/cccl docs/python/ has no coop page
• https://nvidia.github.io/cccl/unstable/python/ lists cuda.compute alone

Why this matters beyond the stale entry

The entry renders into the sidebar of every generated page, so it fails the
lychee step of Docs / Build docs with 83 errors per run, on every open pull
request. main does not catch it because that step only runs on pull requests,
and pre-commit.ci does not catch it because lychee is in the ci: skip list.

Changes

Five deletions across three files, no additions:

• README.md: the cuda.coop bullet
• cuda_python/DESCRIPTION.rst: the cuda.coop bullet
• cuda_python/docs/source/index.rst: the bullet, the link target, and the
  toctree entry

Validation

pre-commit run lychee on the three files, before and after:

# before
[404] https://nvidia.github.io/cccl/unstable/python/coop.html (at 13:15)   DESCRIPTION.rst
[404] https://nvidia.github.io/cccl/unstable/python/coop.html (at 24:16)   index.rst
[404] https://nvidia.github.io/cccl/unstable/python/coop.html (at 55:15)   index.rst
[404] https://nvidia.github.io/cccl/unstable/python/coop.html (at 8:3)     README.md
🔍 65 Total  🔗 20 Unique  ✅ 61 OK  🚫 4 Errors

# after
lychee...................................................................Passed

pre-commit run lychee --all-files also passes across all 198 authored .md
and .rst files, so no other dead link remains in the sources.

Also run and passing on the three files: check-spdx, end-of-file-fixer,
trailing-whitespace, mixed-line-ending, rst-backticks,
rst-directive-colons, rst-inline-touching-normal, check-case-conflict,
check-merge-conflict. git diff --check is clean.

docutils 0.23 on DESCRIPTION.rst reports the same single pre-existing
duplicate-target INFO before and after, and no new messages. On index.rst the
only change in docutils output is the removal of the deleted toctree line.

Notes for the reviewer

• Titled [doc-only] so the test matrix is skipped. The doc job still runs and
  is still required by the aggregator, so the link fix is verified by CI.
• CCCL now describes cuda.compute as device-level primitives, which is close to
  the role the deleted cuda.coop bullet described. I left our cuda.compute
  wording alone to keep this to the broken link. Happy to update it here if you
  would rather it be one change.
• The sibling cuda.compute link also points under unstable/. It currently
  resolves, so I left it, but it carries the same risk of breaking on an upstream
  docs move.
• Needs an assignee and a milestone from a maintainer for pr-metadata-check.

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[mdboom]

/ok to test df9ed66

[github-actions]

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