Docs: fix low quality image

[jamesdanielwhitford]

This adds a higher quality image to avoid blurred text on 4k/retina screens.

See the login-page.png on the website and in the docs.

I have only done one for now to see if it's valuable, but we're big fans of Cloudify at Ritza.co and would love to help out more with the docs/blog or any other technical content if you need! We can run through and update all the images and/or create new pages.

Let me know :)