Add Delete Identity examples

[dabeeeenster]

We get asked a fair bit how to delete identities via the API. Lets add these examples to the appropriate page.

To add here: https://docs.flagsmith.com/clients/rest#code-examples

Edge:
DELETE /api/v1/environments/{environment_api_key}/edge-identities/{identity_uuid}/
Core:
DELETE /api/v1/environments/{environment_api_key}/identities/{id}/