New auth model docs

[infomiho]

Docs changes required for the auth model changes done in #1583

• Removed the old auth model mentions
• Described the new auth model in a separate page
• Updated the texts for auth methods
• Added mentions of the new auth model in relevant places

Left to do

• Go though the tutorial
• Go through each auth provider one more time
• Add a section on the way to use the user
  • Server
  • Client
  • New helpers to get the e.g. username

[Martinsos]

@infomiho nice job all together, I think this is in good shape, I left some comments, some are on docs, some on Auth in general due to me understanding some stuff better, check it out.

[Martinsos]

Approved! There are some conflicts, due to me merging release with the updated docs into the main. I had to fix some links, so that is probably the cause of the conflicts. If you are not sure how to solve them, let me know, I can help, although it should be relatively simple.