OSS: Open /devdocs/start the first time a logged in user opens the dev environment

[nb]

Mockup by @folletto:

• If a user is logged in and opens the development environment for the first time, we should redirect them to /devdocs/start
• We should encourage them to go through the guide (I am working on updates there), not to the “Hello, World!” directly
• As @mtias suggested, the preferences module might be a good fit for persistence
• This task doesn’t include the visual updates to DevDocs

[nb]

Fixed in #482 and #523.