Remove fundraiser invitation endpoint documentation

The fundraiser invitations endpoint is no longer supported.
This removes the schema definition for the fundraiser_invitation resource.

The generated documentation files (source/schema.json and
source/includes/resources.md) will be automatically regenerated
by the CI/CD pipeline when this is merged.

Fixes #22

Co-authored-by: Matt Rohrer <prognostikos@users.noreply.github.com>

Author: github-actions[bot]