[SPARK-31907][DOCS][SQL] Adding location of SQL API documentation

[dominikgehl]

What changes were proposed in this pull request?

Linking to location of SQL API documentation, making it easier and quicker to find it.

Why are the changes needed?

documentation clarity

Does this PR introduce any user-facing change?

No

How was this patch tested?

Only documentation change

[SparkQA]

Kubernetes integration test unable to build dist.

exiting with code: 1
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/45832/

[SparkQA]

Kubernetes integration test starting
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/45836/

[SparkQA]

Kubernetes integration test starting
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/45838/

[SparkQA]

Kubernetes integration test status success
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/45836/

[SparkQA]

Kubernetes integration test status success
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/45838/

[SparkQA]

Test build #141316 has finished for PR 33435 at commit 68882fa.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[srowen]

Merged to master

[SparkQA]

Test build #141321 has finished for PR 33435 at commit 6856a56.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #141323 has finished for PR 33435 at commit dc3a695.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.