Add Substack icon as part of links sections

[robertosilviu]

It could be useful to have the support for substack icon inside the section links of languages.en.toml file. If i add a substack link to my profile it shows an invisible icon. Thank you for this awesome theme.

[leonidasv]

You can add your own custom icons to your assets/icons folder: https://jpanther.github.io/congo/samples/icons/

After that, you can use the icon filename in the languages.en.toml just as you do for built-in icons.

Tip: if you want your icon to work seamlessly for both dark and white colorschemes, add fill="currentColor" to SVG's path. This is not documented AFAIK, I've discovered by myself. Maybe it should be added to the documentation...