Update legal and add OpenJS foundation information

[matthiasrohmer]

As per ampproject/wg-outreach#60.

The foundation's logo from openjs-foundation/artwork needs to be added to the footer. Additionally links to legal documents need to be updated to the following:

<p>Copyright <a href="https://openjsf.org">OpenJS Foundation</a> and AMP Project contributors. All rights reserved. The <a href="https://openjsf.org">OpenJS Foundation</a> has registered trademarks and uses trademarks. For a list of trademarks of the <a href="https://openjsf.org">OpenJS Foundation</a>, please see our <a href="https://trademark-policy.openjsf.org">Trademark Policy</a> and <a href="https://trademark-list.openjsf.org">Trademark List</a>. Trademarks and logos not indicated on the <a href="https://trademark-list.openjsf.org">list of OpenJS Foundation trademarks</a> are trademarks&trade; or registered&reg; trademarks of their respective holders. Use of them does not imply any affiliation with or endorsement by them.</p>
<p><a href="https://openjsf.org">The OpenJS Foundation</a> | <a href="https://terms-of-use.openjsf.org">Terms of Use</a> | <a href="https://privacy-policy.openjsf.org">Privacy Policy</a> | <a href="https://bylaws.openjsf.org">OpenJS Foundation Bylaws</a> | <a href="https://trademark-policy.openjsf.org">Trademark Policy</a> | <a href="https://trademark-list.openjsf.org">Trademark List</a> | <a href="https://www.linuxfoundation.org/cookies">Cookie Policy</a></p>

[tobie]

I don’t think the logo necessarily has to go in the footer, btw.