### What changes were proposed in this pull request?
This PR proposes to add a migration guide for legacy Koalas users in pandas API on Spark.
### Why are the changes needed?
For easier migration.
### Does this PR introduce _any_ user-facing change?
Yes, this adds a new page for migration from Koalas.
### How was this patch tested?
Manually built the docs and checked manually.
Closes#33050 from HyukjinKwon/SPARK-35301.
Authored-by: Hyukjin Kwon <gurwls223@apache.org>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>
