Add maskable logo image

Lighthouse gave us feedback
that we don't have a maskable icon in the manifest file. [1]
We created the icon according to the instructions. [2]
This should please mobile users.

[1]: https://developer.chrome.com/docs/lighthouse/pwa/maskable-icon-audit/
[2]: https://web.dev/articles/maskable-icon#are_my_current_icons_ready

manifest.json

@@ -14,7 +14,14 @@
  {
  "src": "https://blog.fractalsoft.org/assets/images/fractalsoft/blog.svg",
  "type": "image/svg+xml",
- "sizes": "192x192 512x512"
+ "sizes": "192x192 512x512",
+ "purpose": "any"
+ },
+ {
+ "src": "https://blog.fractalsoft.org/assets/images/fractalsoft/blog.svg",
+ "type": "image/svg+xml",
+ "sizes": "192x192 512x512",
+ "purpose": "maskable"
  },
  {
  "src": "https://blog.fractalsoft.org/assets/images/fractalsoft/blog-192x192.png",