Add citations guidance to /help/pro

[garethrees]

Providing built in documentation of how to add citations would allow us to better promote the feature within the app.

For example, we could have a permanent banner on the pro dashboard, and/or reference it in transactional emails.

We have two blog posts that we could use for the majority of the content:

• https://www.mysociety.org/2024/08/01/tell-us-how-youve-used-batch-requests-get-free-pro-credit/
• https://www.mysociety.org/2020/01/14/has-your-foi-request-been-used-in-a-news-story-now-you-can-let-everyone-know/