doc: Ensure searchtools.js gets included

Searchtools did not get included with Debian > stretch and
Sphinx > 1.6.7.

Sphinx 3.4.0 also needs `language_data.js` to properly search.
See readthedocs/sphinx_rtd_theme#1021

`searchtools.js` and `language_data.js` are only needed by
`search.html`. So maybe better just add `search.html` to the template.

Forwarded: no
Signed-off-by: Benjamin Drung <benjamin.drung@ionos.com>

doc/_themes/saltstack/layout.html

@@ -27,6 +27,12 @@
 {% set script_files = [
     '_static/js/vendor/bootstrap.min.js',
 ] + script_files %}
+{%- if not '_static/searchtools.js' in script_files %}
+{%- set script_files = script_files + ['_static/searchtools.js'] %}
+{%- endif %}
+{%- if not '_static/language_data.js' in script_files %}
+{%- set script_files = script_files + ['_static/language_data.js'] %}
+{%- endif %}
 
 {%- macro relbar() %}
     <div class="related">