Docs missing clear path to instructions for doc contributions

[huertanix]

Description

At the moment, there are no clear steps for potential documentation contributors to make/view/send changes.

In https://docs.securedrop.org/en/latest/development/contributing.html#writers the language links to issues, but the actual instructions to get started and set up a dev environment are hidden in the Documentation Guidelines page for some reason (https://docs.securedrop.org/en/latest/development/documentation_guidelines.html) rather than as part of a page on getting the docs environment called out via some sort of next step verbiage. The whole page shares a common "Next" button which also adds to the confusion as the "Next" page is the same no matter whether you're coming in as a dev, translator or writer.

User Stories

As a potential contributor, I would like to have a clear path to contributing to SecureDrop's documentation.