Add SVG icon and more PNG icon sizes to assets

The previous icons had sizes that were looked really pixelated, at least
on GNOME's Activities page for instance.

Also update the packaging docs for the SVG icon and make them clearer and
more specific.

book/src/cli-installation.md

@@ -137,6 +137,10 @@ If this variable is set during compilation, the specified path will be compiled
 In order to test that everything is working as intended, you can open `numbat` and type `let meter = 2`. The
 path in the error message should point to the specified location (and *not* to `<builtin>/…`).
 
-If your OS uses `.desktop` files, you should probably also install `assets/numbat.desktop` and `assets/numbat-*x*.png` to their
-appropriate locations (typically `/usr/share/applications` and `/usr/share/icons` respectively) so that users can e.g. pin
-Numbat to GNOME's Dash.
+If your OS uses `.desktop` files, you should probably also install:
+
+- `assets/numbat.desktop` (typically to `/usr/share/applications`)
+- `assets/numbat.svg` (typically to `/usr/share/icons/hicolor/scalable/apps`)
+- `assets/numbat-*x*.png` (typically to e.g. `/usr/share/icons/hicolor/32x32/apps`, depending on each icon's size)
+
+This allows users to e.g. pin Numbat to GNOME's Dash.