[SPARK-21069][SS][DOCS] Add rate source to programming guide.
[SPARK-21069][SS][DOCS] Add rate source to programming guide.

## What changes were proposed in this pull request?

SPARK-20979 added a new structured streaming source: Rate source. This patch adds the corresponding documentation to programming guide.

## How was this patch tested?

Tested by running jekyll locally.

Author: Prashant Sharma <prashant@apache.org>
Author: Prashant Sharma <prashsh1@in.ibm.com>

Closes #18562 from ScrapCodes/spark-21069/rate-source-docs.

(cherry picked from commit d0bfc6733521709e453d643582df2bdd68f28de7)
Signed-off-by: Shixiong Zhu <b59cd5ce8cb5ae44d26fc6984be1f84cf74771a7@databricks.com>

jlopezmalla/spark,jlopezmalla/spark,jlopezmalla/spark,jlopezmalla/spark,jlopezmalla/spark,jlopezmalla/spark,jlopezmalla/spark