Update semgrep to run for javascript

* Update semgrep analyzer in SAST.gitlab-ci.yml to trigger for
  JavaScript, TypeScript, and React files.
* Update docs to mention that semgrep can work for JavaScript,
  TypeScript, and React

changelogs/unreleased/js-semgrep.yml

+---
+title: Add JavaScript, TypeScript, and React support to the semgrep analyzer.
+merge_request: 55257
+author:
+type: added

lib/gitlab/ci/templates/Security/SAST.gitlab-ci.yml

@@ -303,6 +303,10 @@ semgrep-sast:
           $SAST_EXPERIMENTAL_FEATURES == 'true'
       exists:
         - '**/*.py'
+        - '**/*.js'
+        - '**/*.jsx'
+        - '**/*.ts'
+        - '**/*.tsx'
 
 sobelow-sast:
   extends: .sast-analyzer