### What changes were proposed in this pull request?
Document SELECT statement in SQL Reference Guide. In this PR includes the main
entry page for SELECT. I will open follow-up PRs for different clauses.
### Why are the changes needed?
Currently Spark lacks documentation on the supported SQL constructs causing
confusion among users who sometimes have to look at the code to understand the
usage. This is aimed at addressing this issue.
### Does this PR introduce any user-facing change?
Yes.
**Before:**
There was no documentation for this.
**After.**
<img width="972" alt="Screen Shot 2020-01-19 at 11 20 41 PM" src="https://user-images.githubusercontent.com/14225158/72706257-6c42f900-3b12-11ea-821a-171ff035443f.png">
<img width="972" alt="Screen Shot 2020-01-19 at 11 21 55 PM" src="https://user-images.githubusercontent.com/14225158/72706313-91d00280-3b12-11ea-90e4-be7174b4593d.png">
<img width="972" alt="Screen Shot 2020-01-19 at 11 22 16 PM" src="https://user-images.githubusercontent.com/14225158/72706323-97c5e380-3b12-11ea-99e5-e7aaa3b4df68.png">
### How was this patch tested?
Tested using jykyll build --serve
Closes#27216 from dilipbiswal/sql_ref_select_hook.
Authored-by: Dilip Biswal <dkbiswal@gmail.com>
Signed-off-by: Sean Owen <srowen@gmail.com>
## What changes were proposed in this pull request?
This is a initial PR that creates the table of content for SQL reference guide. The left side bar will displays additional menu items corresponding to supported SQL constructs. One this PR is merged, we will fill in the content incrementally. Additionally this PR contains a minor change to make the left sidebar scrollable. Currently it is not possible to scroll in the left hand side window.
## How was this patch tested?
Used jekyll build and serve to verify.
Closes#25459 from dilipbiswal/ref-doc.
Authored-by: Dilip Biswal <dbiswal@us.ibm.com>
Signed-off-by: gatorsmile <gatorsmile@gmail.com>
