docs: add contributor guide for bringing up a new Spark version

[andygrove]

Which issue does this PR close?

N/A. Documentation-only addition; no tracking issue.

Rationale for this change

Bringing up a new Apache Spark version in Comet (recent examples: 4.0, 4.1, 4.2-preview) touches the build, the shim layer, CI, and three different test surfaces (Comet's JVM tests, Spark's SQL tests, and TPC-DS plan stability). When that work is bundled into one PR the diff is hard to review and harder to bisect, and contributors who have not done a Spark bring-up before have to reverse-engineer the recipe from prior PRs.

This page captures the recipe explicitly so the next bring-up can follow a known path.

What changes are included in this PR?

• New page docs/source/contributor-guide/adding_a_new_spark_version.md describing a staged-PR workflow:
  1. PR 1: Maven profile, shims (including the spark-4.x shared / spark-X.Y per-version layering), version-detection helper, compile-only CI job.
  2. PR 2: Enable Comet's JVM tests under the new profile; skip failing tests with assume() calls linked to GitHub issues.
  3. PR 3: Enable Spark SQL tests and plan stability under the new profile; same skip-with-linked-issue rule.
  4. Follow-up PRs: one issue (or one cluster) per PR, removing the corresponding skip as part of the fix.
• Cross-reference added to docs/source/contributor-guide/spark-sql-tests.md so contributors who land on the existing diff-creation section get pointed at the broader staged plan first.
• New page added to the contributor guide toctree in index.md.
• The diff-creation mechanics already covered in spark-sql-tests.md are intentionally not duplicated; the new page links to that section instead.

How are these changes tested?

Documentation only; no code changes. Verified the toctree entry renders by inspecting index.md and confirmed the cross-references resolve to existing files in the contributor guide.