Restructure Jenkins setup documentation

[b-fein]

Motivation

Right now the documentation jumps frequently between steps required only for local setups, then steps required for production setups, some Nginx setup in between. This is very hard to understand.

Goal

The documentation should be restructured to have a dedicated sections for

• Jenkins setup in a container for dev
• Jenkins production setup
• Additional production tasks like build agent setup, …

Originally posted by @b-fein in #9266 (comment)

[dfuchss]

@b-fein @BaumiCoder I can have a look at and can also help with a draft :)

[krusche]

Good idea 👍