You should be able to write that as a test and then go deeper as you get past the 1st few hurdles. Github should have samples that you can pull in and modified to fit your env. I would not do a major change like policy add|delete but would start with low hanging fruits likes add/addrgrp/admin/global settings etc.....
FWIW: Operational practice you don't put the password in the playbook but for testing and to ensure it's working I do, but that's just me.
Just ensure you run ANSIBLE_DEBUG and -vvv for more verbose details and work thru your issues. It always boils down to either;
1: wrong credentials
3: bad directives in the playbook
4: or the host can NOT reach the fortigate[/ul]
But the samples show in the yml files is what I used for testing an verification with fortios. This works for 6.2.x and 6.4.x versions btw.
YMMV but the above links in the blog-post will get you in the right direction and handing into the right port. Also call out a simple test with "curl" if you want to test the api-user. Again samples in the blog post.
Look at Ansible Galaxy... there is a collection of FortiOS and Fortimanager modules available for download. If you're already figured that out, great. The documentation isn't the greatest. Struggling through some things with it myself.