docs: add link to typescript virtuals docs

Author: vkarpov15

docs/layout.pug

@@ -111,6 +111,8 @@ html(lang='en')
                           a.pure-menu-link(href=`${versions.versionedPath}/docs/typescript/query-helpers.html`, class=outputUrl === `${versions.versionedPath}/docs/typescript/query-helpers.html` ? 'selected' : '') Query Helpers
                         li.pure-menu-item.sub-item
                           a.pure-menu-link(href=`${versions.versionedPath}/docs/typescript/populate.html`, class=outputUrl === `${versions.versionedPath}/docs/typescript/populate.html` ? 'selected' : '') Populate
+                        li.pure-menu-item.sub-item
+                          a.pure-menu-link(href=`${versions.versionedPath}/docs/typescript/virtuals.html`, class=outputUrl === `${versions.versionedPath}/docs/typescript/virtuals.html` ? 'selected' : '') Virtuals
                         li.pure-menu-item.sub-item
                           a.pure-menu-link(href=`${versions.versionedPath}/docs/typescript/subdocuments.html`, class=outputUrl === `${versions.versionedPath}/docs/typescript/subdocuments.html` ? 'selected' : '') Subdocuments
               li.pure-menu-item