### What changes were proposed in this pull request?
This PR intends to add a dedicated page for SQL-on-file in SQL documents.
This comes from the comment: https://github.com/apache/spark/pull/30095/files#r508965149
### Why are the changes needed?
For better documentations.
### Does this PR introduce _any_ user-facing change?
<img width="544" alt="Screen Shot 2020-10-28 at 9 56 59" src="https://user-images.githubusercontent.com/692303/97378051-c1fbcb80-1904-11eb-86c0-a88c5269d41c.png">
### How was this patch tested?
N/A
Closes#30165 from maropu/DocForFile.
Authored-by: Takeshi Yamamuro <yamamuro@apache.org>
Signed-off-by: Takeshi Yamamuro <yamamuro@apache.org>
