Message Priority / :master docker image and other improvements.

[jmattheis]

• Adds two pages for message priorities and the gotify/server:master docker image
• Run the prompt with Claude "Inspect with a sub agent every page and try to simplify the pages and fix possible issues.". For the most part this improved the structuring of the sentences, and removed some fluff from the docs. I think it's an improvement to before, but I'm okay with dropping this commit.
• Reorder the navigation.

Fixes #101
Fixes gotify/server#907

@eternal-flame-AD can you have a glance over this?