[MINOR][DOC] Remove obsolete `contributing-to-spark.md`
[MINOR][DOC] Remove obsolete `contributing-to-spark.md`

### What changes were proposed in this pull request?

This PR removes obsolete `contributing-to-spark.md` which is not referenced from anywhere.

### Why are the changes needed?

Just clean up.

### Does this PR introduce _any_ user-facing change?

No. Users can't have access to contributing-to-spark.html unless they directly point to the URL.

### How was this patch tested?

Built the document and confirmed that this change doesn't affect the result.

Closes #33619 from sarutak/remove-obsolete-contribution-doc.

Authored-by: Kousuke Saruta <7d45abe39fd242ee2f6575da28f13d66568418b3@oss.nttdata.com>
Signed-off-by: Hyukjin Kwon <2680965ad5aae29275c8fb10150cb10f5554e78b@apache.org>
(cherry picked from commit c31b653806f19ffce018651b6953bf47d019d7e8)
Signed-off-by: Hyukjin Kwon <2680965ad5aae29275c8fb10150cb10f5554e78b@apache.org>

milliman/spark,milliman/spark,milliman/spark,milliman/spark,milliman/spark,milliman/spark,milliman/spark