[enhancement]: Add a self-harm category

[sarayourfriend]

What

We got a request via the form to add a self-harm category. This is probably a good opportunity to make it possible for resources to fall under multiple categories somehow.

Why

It's hard to find the self-harm specific resources at the moment.

[sarayourfriend]

The work to make resources able to be in multiple categories is already done. This issue just needs someone to pick it up and add select resources to a new self-harm category.