Introductory material is broken or behind sign-up walls

[dogweather]

Is your feature request related to a problem? Please describe.

Use case: as a potential user/contributor, I want to get an architectural and UX overview of Rails Admin.

However, I wasn't able to:

• The demo instance on Heroku linked from the README is crashed and doesn't restart.
• The Overview and Best Practices slide deck is hosted on an incredibly ad-choked platform. In order to download the file, registration is necessary.

Describe proposed solution(s)

• Make the slide deck easily downloadable via a link from the README.
• Fix or remove the link to the demo instance.

[codealchemy]

The issue with the demo not working is a dupe of #3537

Re: the slide deck, it's also 10+ years old - I'm curious how relevant it remains given the changes since then, and if the content is otherwise already covered in READMEs / the Wiki (or may need to be added there).