Adding a new subproject

When a new repository is added to the Matplotlib organization with

documentation (or an existing repository adds documentation), it will be

necessary to re-configure the server to serve those files. Note, it is

currently assumed that the documentation is on the `gh-pages` branch of the

repository, and it will be served from the top-level subdirectory with the same

name as the repository (similar to GitHub Pages.) There are 4 steps to achieve

this:

1. Generate a secret to secure the webhook. You can follow [GitHub's

instructions for creating

one](https://docs.github.com/en/developers/webhooks-and-events/webhooks/securing-your-webhooks).

2. Add repository to Ansible:

1. Add an entry to the `repos` variable at the top of `matplotlib.org.yml`.

2. Add the webhook secret to `files/webhook_vars.yml`.

3. Re-run Ansible on the playbook like [below](#running-ansible). This should

clone the new repository and update the webhook handler.

4. Configure a webhook on the new repository with the following settings:

- Payload URL of `https://do.matplotlib.org/gh/<repository>`

- Content type of application/json

- Use the secret generated in step 1

- Trigger only on "push" events

If everything is done correctly, the GitHub webhook should have posted an

initial "ping" event successfully, and documentation should be available at

`https://matplotlib.org/<repository>`.