[DOC] Document cuDF spilling

[nethum529]

Description

Closes #13161.

Adds a cuDF spilling user guide that explains setup and limit behavior.
Lists each public option and links the page from the cuDF guide.

Verification: pre-commit run --files docs/cudf/source/cudf/spilling.md docs/cudf/source/cudf/index.md

Checklist

• I am familiar with the Contributing Guidelines.
• Tests are unchanged because this PR changes documentation only.
• The documentation is up to date with these changes.

[copy-pr-bot]

This pull request requires additional validation before any workflows can run on NVIDIA's runners.

Pull request vetters can view their responsibilities here.

Contributors can view more details about this message here.

[nethum529]

Could a maintainer add the doc and non-breaking labels? The Label Checker requires both categories, and external contributors cannot apply repository labels.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Enterprise

Run ID: 27c746b8-ff8d-4570-8dab-a5a31988a62b

📥 Commits

Reviewing files that changed from the base of the PR and between 8831e6a and 5ccbf7c.

📒 Files selected for processing (1)

• docs/cudf/source/cudf/spilling.md

🚧 Files skipped from review as they are similar to previous changes (1)

• docs/cudf/source/cudf/spilling.md

---

📝 Walkthrough

Summary by CodeRabbit

• Documentation
  • Added a cuDF user guide covering GPU memory spilling to host memory.
  • Documented how to enable spilling through configuration or environment variables.
  • Added details on spill-related options, defaults, ordering, device limits, automatic unspilling, and limitations.

Walkthrough

Changes

Spilling documentation

Layer / File(s)	Summary
Spilling concepts and configuration docs/cudf/source/cudf/spilling.md, docs/cudf/source/cudf/index.md	Adds the spilling guide, enablement instructions, configuration options, environment variables, defaults, and index navigation entry.
Spilling behavior and limits docs/cudf/source/cudf/spilling.md	Documents least-recently-accessed ordering, automatic unspilling, transfer costs, and soft device-limit behavior.

Estimated code review effort: 1 (Trivial) | ~5 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely summarizes the main change: documenting cuDF spilling.
Description check	✅ Passed	The description is directly related to the documentation-only spilling guide and its link in the cuDF docs.
Linked Issues check	✅ Passed	The PR documents cuDF's spilling option and how to enable it, matching issue #13161's request.
Out of Scope Changes check	✅ Passed	The added spilling guide and index link are aligned with the stated documentation objective and do not appear unrelated.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

---

_{Comment @coderabbitai help to get the list of available commands.}

[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@docs/cudf/source/cudf/spilling.md`:
- Around line 40-41: Update the spill_stats entry in the configuration table to
state that level 1 reports spill duration and bytes spilled, and that level 2 or
higher records tracebacks for permanently exposed buffers. Preserve the existing
environment variable, default value, and surrounding documentation.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Enterprise

Run ID: dcbd3b9a-22a6-4c98-8a65-47135f579e9f

📥 Commits

Reviewing files that changed from the base of the PR and between 2cfb92e and 8831e6a.

📒 Files selected for processing (2)

• docs/cudf/source/cudf/index.md
• docs/cudf/source/cudf/spilling.md