refactor: migrate to material symbols icon set

ICONS.md

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+# Icons
+
+A few notes to our icon set, for users and future maintainers.
+
+## Usage
+
+To find icon names, visit the
+[Google Material Symbols Page](https://fonts.google.com/icons?icon.style=Rounded)
+
+We have extended the `ion-icon` element via a directive. **Make sure your
+module imports the `IonIconModule`**, you can then proceed using `ion-icon`s
+as usual.
+
+The modified `ion-icon` comes with a few extra features:
+
+* `[fill]` controls the fill color of the icon.
+* `[weight]` controls the font weight of the icon.
+* `[size]` controls the font size of the icon.
+* `[grade]` controls the font grade of the icon.
+
+All of these attributes are animated as described
+[here](https://developers.google.com/fonts/docs/material_symbols).
+
+![](/readme-resources/fill-axis.gif)
+
+You can also control these attributes via css:
+
+```scss
+ion-icon ::ng-deep stapps-icon {
+  --fill: 1;
+  --grade: 0;
+  --weight: 400;
+}
+```
+
+Sometimes icon code points cannot be determined automatically, for whatever
+reason. In this case, you will need to specify the code point manually in
+the config file.
+
+### Icon Font Minification
+
+Icon font minification is done automatically, but requires you to
+follow a few simple rules:
+
+1. Use the tagged template literal for referencing icon names in
+   TypeScript files and code
+```ts
+SCIcon`icon_name`
+```
+2. When using `ion-icon` in HTML, reference either icons that went through
+   the `SCIcon` tag or write them as one of the following:
+```html
+<!-- do -->
+<ion-icon name="icon_name"></ion-icon>
+<ion-icon [name]="'icon_name'"></ion-icon>
+<!-- don't -->
+<ion-icon name="icon_name"/> <!-- self-closing -->
+<ion-icon [name]="condition ? 'icon_name' : 'other_icon_name'"></ion-icon>
+```
+
+Icons that are unknown at compile time can be specified in the
+`additionalIcons` property of the `icons.config.ts` file.
+
+The minification can then be done by running
+```shell
+npm run minify-icons
+```
+
+Unfortunately, I was unable to find a JS package that could to the job,
+and had to rely on the Python module [fonttools](https://github.com/fonttools/fonttools).
+
+That means that you might run into additional issues when running the
+above-mentioned command.
+