Provide logos/press kit

[grobie]

There are many cases were people want to embed the prometheus logo somewhere, e.g. in third-party integrations. We should provide a logo pack somewhere in different resolutions.

@juliusv

[ahus1]

Ideas:

• add logo section to introduction/media
• add a link to the cloudnative store - although I have to say shipping takes very long and is expensive
• add a link to stickermule so people can order stickers themselves
• add entry to FAQ

I'm happy to provide a pull request given some comments on the options above.