### What changes were proposed in this pull request?
This PR proposes to merge contents and remove obsolete pages in Development section, especially about pandas API on Spark.
Some were removed, and some were merged to the existing PySpark guides. I will inline some comments in the PRs to make the review easier.
### Why are the changes needed?
To guide developers on the code base of pandas API on Spark.
### Does this PR introduce _any_ user-facing change?
Yes, it updates the user-facing documentation.
### How was this patch tested?
Manually built the docs and checked.
Closes#32926 from HyukjinKwon/SPARK-35644.
Authored-by: Hyukjin Kwon <gurwls223@apache.org>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>
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