### What changes were proposed in this pull request?
This PR is to add a redirect to sql-ref.html.
### Why are the changes needed?
Before Spark 3.0 release, we are using sql-reference.md, which was replaced by sql-ref.md instead. A number of Google searches I’ve done today have turned up https://spark.apache.org/docs/latest/sql-reference.html, which does not exist any more. Thus, we should add a redirect to sql-ref.html.
### Does this PR introduce _any_ user-facing change?
https://spark.apache.org/docs/latest/sql-reference.html will be redirected to https://spark.apache.org/docs/latest/sql-ref.html
### How was this patch tested?
Build it in my local environment. It works well. The sql-reference.html file was generated. The contents are like:
```
<!DOCTYPE html>
<html lang="en-US">
<meta charset="utf-8">
<title>Redirecting…</title>
<link rel="canonical" href="http://localhost:4000/sql-ref.html">
<script>location="http://localhost:4000/sql-ref.html"</script>
<meta http-equiv="refresh" content="0; url=http://localhost:4000/sql-ref.html">
<meta name="robots" content="noindex">
<h1>Redirecting…</h1>
<a href="http://localhost:4000/sql-ref.html">Click here if you are not redirected.</a>
</html>
```
Closes#28914 from gatorsmile/addRedirectSQLRef.
Authored-by: gatorsmile <gatorsmile@gmail.com>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>
## 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>
