[SPARK-31333][FOLLOWUP][DOC] Link Join Hints doc in SQL perf tuning guide
[SPARK-31333][FOLLOWUP][DOC] Link Join Hints doc in SQL perf tuning guide

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

This is a follow-up of https://github.com/apache/spark/pull/28113.
There is also a brief section about Join hints in SQL perf tuning guide: https://spark.apache.org/docs/latest/sql-performance-tuning.html . We should link the new Join hint doc in it.

### Why are the changes needed?

So that users can read more examples.

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

No

### How was this patch tested?

Manually build the doc and check it:
![image](https://user-images.githubusercontent.com/1097932/78860030-f7cb7800-79e5-11ea-8573-c0587d43a7dc.png)

Closes #28161 from gengliangwang/joinHintFollowUp.

Authored-by: Gengliang Wang <bfe1a7b36425bd89859a70a36586361d28ec35eb@databricks.com>
Signed-off-by: HyukjinKwon <2680965ad5aae29275c8fb10150cb10f5554e78b@apache.org>

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