[Pre-Draft][WIP] Started rewriting Security component docs

[wouterj]

Pfew 😓

Warning: This PR is in very rough shape. It's written in one go, without reviewing what I've written. You can read it, but please don't waste your time doing detailed reviews and only review it very globally.

The current component docs have been written a long time ago. During the years, lots of people have struggled with the Security component. The community has tried to solve this by explaining the component in different ways (like a talk, a book, etc.). We've learned which way of explaining people like. Now it's time to improve the official docs!

The Goals

• Always show the big picture; what's involved in having a working security system
• Clearly show where one should extend/customize the system and were one shouldn't
• Show how to use the component standalone, including its sub-components.

Todo

• Write Core/Authorization article
• Write HTTP article (which basically is about Authentication)
• Reread... tweak... reread... review... tweak
• Remove old component docs

PR Info Table

Q	A
Fixed docs?	yes
New docs?	yes (no new features though)
Applies to	2.3+
Fixed tickets	At least #5289, #4759

[javiereguiluz]

This paragraph needs some additional explanation about how can this auth provider possibly exist. Why do we need to authenticate anonymous users?

[weaverryan]

@wouterj I'm closing this PR because it's old and stalled. See #7496