Destroy dog mutation

[alexander-mathieu]

This PR adds a destroyDog mutation, used to delete puppies from your life!

Changes proposed in this pull request:

• Adds destroyDog mutation and spec
• Adds destroyDog mutation to the GraphQL schema
• Adds destroyDog documentation to the README

Resolves #78

The following checks have been completed:

• [x] Tested my new feature(s) as well as any feasible edge cases (if possible)
• [x] Checked coverage/index.html - did not add any new code that's not covered by testing (if possible)
• [x] Merged in the latest master to my branch with git pull origin master & resolved merge conflicts
• [x] Ran rails db:migrate
• [x] Ran the test suite - all tests are passing (or maybe skipped)
• [x] Checked affected endpoints in Postman / GraphiQL
• [x] Updated README for changes (new endpoints, new gems, etc)