Add confirmation endpoints and instructions to documentation

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](https://devise-token-auth.gitbook.io/devise-token-auth/usage) 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.

Start with the Usage page and the existing Reset password flow guide, then review the related merged pull request #1343 to understand the unreleased confirmation behavior. Document the three listed /confirmation endpoints and add a Confirm email flow guide explaining how to implement :confirmable in new or existing applications.

Written by the indexing model from the issue text.