Add link to logos

[nilsvu]

Proposed changes

Upgrade instructions

Code review checklist

• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
  Then open index.html.
• The code follows the stylistic and code quality guidelines listed in the
  code review guide.
• The PR lists upgrade instructions and is labeled bugfix or
  new feature if appropriate.

Further comments

[knelli2]

Should the word "spectre" be a bit larger? It looks a tad small in the space that it's in.

[nilsvu]

🤷‍♂️ I think it's fine, it's personal preference I suppose. Try this size and adjust later if we find it's too small?

FYI, I intend to add a gravitational wave visualization to this banner when I find the time, to have the "s" radiate a burst of GWs in 3D.

[knelli2]

I think we'll want to change how this looks for dark-mode. The random white rectangle looks a bit odd. But we can do that later.