## 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>
