[SPARK-38186][SQL] Improve the README of Spark docs
[SPARK-38186][SQL] Improve the README of Spark docs

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

Improve https://github.com/apache/spark/blob/master/docs/README.md, mark some of the setup steps as optional.
Also, recommend developers to use `SKIP_API=1` if no API docs are needed.

### Why are the changes needed?

 Developers usually need to generate HTML Docs without API Docs. Marking the setup for SQL/Python/R doc as optional can help them avoid unnecessary efforts.

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

No

### How was this patch tested?

Manual test

Closes #35491 from gengliangwang/updateDoc.

Authored-by: Gengliang Wang <789196f1837e809005c9b5b207ae21d5a78ca970@apache.org>
Signed-off-by: Hyukjin Kwon <2680965ad5aae29275c8fb10150cb10f5554e78b@apache.org>

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