[FIX] *: make several untranslatable parts translatable

Some parts of the documention were currently not translatable:
- the custom `Card` directives
- the strings in the layout HTML pages
- the placeholder text in the search box
- the "Edit on GitHub" text on each page

This commit makes sure that all of these are translatable now.

Part-of: odoo/documentation#15894
Signed-off-by: Tiffany Chang (tic) <tic@odoo.com>
Signed-off-by: Dylan Kiss (dyki) <dyki@odoo.com>

.weblate.json

@@ -61,6 +61,11 @@
                 "filemask": "locale/*/LC_MESSAGES/services.po",
                 "new_base": "locale/sources/services.pot"
             },
+            {
+                "name": "sphinx",
+                "filemask": "locale/*/LC_MESSAGES/sphinx.po",
+                "new_base": "locale/sources/sphinx.pot"
+            },
             {
                 "name": "studio",
                 "filemask": "locale/*/LC_MESSAGES/studio.po",