Change section order on index page

[ckurze]

Summary of the changes / Why this is an improvement

• Reorder sections on index.md to have more importance on the application domains and integrations over reference architectures.

Checklist

• Link to issue this PR refers to (if applicable):
• CLA is signed

[amotl]

Now, when steering into the page, the appearance has a visual bias towards a wall of tiles, which is not optimal. I liked that the image to the reference architecture acted as a kind of a page divider, but sure enough it is only personal aesthetics.

https://cratedb.com/docs/guide/

Maybe a future iteration will do something differently, for example break the page by different means, or by using different styles of tiles, to remedy the monoculture/brutalism flavor of the current representation.