fix(docs): add a section on custom svg paths

[christian-bromann]

In #1302 a user reported an issue experiencing issues loading Ionicons from a custom source. This patch adds a section on how to set a custom resource path through Stencil.

Note: the reported issue used a specific way to integrate ionicons via the @ionic/angular/standalone package. For folks just using an ionicon script tag as described in the readme, this won't be much useful. The added documentation requires the user to use a compiler that resolves the Ionicon and application code with the same resolver.

An alternative would be to add these docs to the @ionic/angular package, please advise!

[sean-perkins]

Small suggestions, non-blocking.

[christian-bromann]

@liamdebeasi @sean-perkins thanks for the feedback. Added changes accordingly.

[liamdebeasi]

Thanks!