docs: mark spdx headers with the default license to website files

Those were added in b41a3f66c9 without
an explicit license, so they are under the default license. Some files
already got a header previously, so this only touches the remaining.

The same should be done for docs/_data/extra_pages.json, but it's json, and
json doesn't allow comments.

docs/_includes/header.html

@@ -1,3 +1,5 @@
+<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
+
 <header class="site-header">
 
   <div class="wrapper">