docs: Markdown logo in the README so it renders on NuGet too

All six packages ship this README, and nuget.org renders Markdown only, so the <picture> block would show as raw HTML there. It is now a Markdown image of assets/readme-logo.svg: the brand tile at 64 px, ink on light and paper on dark through prefers-color-scheme, drawn with md-files brand/generate.py. assets/icon-dark.png is no longer used.

Author: Shewart

README.md

@@ -1,7 +1,4 @@
-<picture>
-  <source media="(prefers-color-scheme: dark)" srcset="https://raw.githubusercontent.com/shellui-dev/shelldocs/main/assets/icon-dark.png" />
-  <img src="https://raw.githubusercontent.com/shellui-dev/shelldocs/main/assets/icon.png" alt="" width="64" />
-</picture>
+![ShellDocs](https://raw.githubusercontent.com/shellui-dev/shelldocs/main/assets/readme-logo.svg)
 
 # ShellDocs