Add confirmation endpoints and instructions to documentation

[experimatt]

This isn't a code-related issue, but a request to clarify email confirmation flows and endpoints in the documentation. I'd be happy to submit a pull request for this, but I need more information about how this is expected to work, before I'll be able to explain it. There's also related code that's been merged but not released yet (specifically: https://github.com/lynndylanhurley/devise_token_auth/pull/1343).

Tasks:

• [ ] Add /confirmation endpoints to the Usage page of the documentation. I believe these are the routes:
  • GET /confirmation/new => devise_token_auth/confirmations#new
  • GET /confirmation => devise_token_auth/confirmations#show
  • POST /confirmation => devise_token_auth/confirmations#create
• [ ] Add a Confirm email flow guide to the documentation. Similar to Reset password flow, but for email confirmations. The goal here is to clearly articulate the expected behavior and how to implement :confirmable in a new or existing application.