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

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

Those were added in b41a3f66c97e3d861faed04b727daf929383b827 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.