Add GitHub sponsors button to docs navbar

[pokey]

Should be top right, to the left of the GitHub link. Can probably use this technique https://github.com/facebook/docusaurus/issues/7227. Should probably use the embed code:

<iframe src="https://github.com/sponsors/pokey/button" title="Sponsor pokey" height="35" width="116" style="border: 0;"></iframe>

• [ ] Also add page with Sponsorship info to docs site