Release other pages with the new theme

[timhoffm]

Not sure where this belongs. All pages should be updated using the recent 3.6.0 release of this theme.

This is at least:

• https://matplotlib.org/
• https://matplotlib.org/mpl-third-party/

[QuLogic]

• Announce 3.6.0 and update theme mpl-brochure-site#61
• Update to 3.6 Sphinx theme mpl-gui#10
• Update to 3.6 Sphinx theme mpl-third-party#134
• Update to 3.6 Sphinx theme governance#32

I did not fix cheatsheets, as that has a more complicated update process, and I guess needs to be fully audited to work with 3.6.

[timhoffm]

All done.