docs(readme): swap the animated logo for the static mark

The header opened on a 162-frame GIF, 878 KB across the light and dark pair for
an animation that runs before anyone has read a line. docs/ now carries the same
mark as the SVG the app itself ships, so the README header and the navbar show
one logo rather than two renderings of it, and the picture element keeps
switching it per colour scheme.

The GIFs stay in docs/ unreferenced, in case the animation is wanted elsewhere.

README.md

@@ -1,9 +1,9 @@
 <div align="center">
 
 <picture>
-  <source media="(prefers-color-scheme: dark)" srcset="docs/logo-trek-light.gif" />
+  <source media="(prefers-color-scheme: dark)" srcset="docs/logo-trek-light.svg" />
-  <source media="(prefers-color-scheme: light)" srcset="docs/logo-trek-dark.gif" />
+  <source media="(prefers-color-scheme: light)" srcset="docs/logo-trek-dark.svg" />
-  <img src="docs/logo-trek-dark.gif" alt="TREK" height="96" />
+  <img src="docs/logo-trek-dark.svg" alt="TREK" height="96" />
 </picture>
 
 <br />