Add beautiful mesh gradient-based background to the docs

[Zingzy]

Use this color: #1E162A

something similar, it should match the brand colors of https://spoo.me, https://docs.spoo.me

See this example:

https://docs.replit.com/getting-started/intro-replit

Refer to the mintlify docs to see on how to add custom css: https://mintlify.com/docs