docs: restored page on roles in security, executed other touchups, and… (#11978)

* Restored docs page on roles in security, executed other touchups, and renamed docs readme.txt to readme.md

* missed a url anchor on the security page

* fixed licensing issue and limited char width to 80

* fixed licensing issue and limited char width to 80

* whitespace experiments

* removed 2 periods

* staging whitespace

* addressed Robs feedback

* implemented robs second round of feedback on public_role_like

docs/src/pages/index.tsx

@@ -468,7 +468,7 @@ const Theme = () => {
                 )}
               </ul>
               <span className="databaseSub">
-                ... and any other
+                ... and many other
                 <a href="https://superset.apache.org/docs/databases/installing-database-drivers">
                   {' '}
                   compatible databases