website/docs: update and rearchitect the oauth2 provider docs

[tanberry]

This PR adds more conceptual info to the index page (thanks Jens), and creates a new page for procedurals (to be written). It also moves all content about GitHub compatibility to a separate child-page.

If applicable

• The documentation has been updated
• The documentation has been formatted (make website)

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[authentik-automation]

Images automagically compressed by Calibre's image-actions ✨

Compression reduced images by 40.8%, saving 31.25 KB.

Filename	Before	After	Improvement	Visual comparison
website/docs/add-secure-apps/providers/oauth2/OAuth2-diagram.png	76.51 KB	45.26 KB	-40.8%	View diff

161 images did not require optimisation.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.64%. Comparing base (1623885) to head (598e6c8).
Report is 41 commits behind head on main.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #11916      +/-   ##
==========================================
- Coverage   92.68%   92.64%   -0.05%     
==========================================
  Files         762      762              
  Lines       38151    38151              
==========================================
- Hits        35362    35344      -18     
- Misses       2789     2807      +18     

Flag	Coverage Δ
e2e	49.05% <ø> (-0.05%)	⬇️
integration	24.76% <ø> (ø)
unit	90.22% <ø> (+<0.01%)	⬆️

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[github-actions]

authentik PR Installation instructions

Instructions for docker-compose

Add the following block to your .env file:

AUTHENTIK_IMAGE=ghcr.io/goauthentik/dev-server
AUTHENTIK_TAG=gh-598e6c8850d2fd1d12f38a54bd93b28c55b491f1
AUTHENTIK_OUTPOSTS__CONTAINER_IMAGE_BASE=ghcr.io/goauthentik/dev-%(type)s:gh-%(build_hash)s

For arm64, use these values:

AUTHENTIK_IMAGE=ghcr.io/goauthentik/dev-server
AUTHENTIK_TAG=gh-598e6c8850d2fd1d12f38a54bd93b28c55b491f1-arm64
AUTHENTIK_OUTPOSTS__CONTAINER_IMAGE_BASE=ghcr.io/goauthentik/dev-%(type)s:gh-%(build_hash)s

Afterwards, run the upgrade commands from the latest release notes.

Instructions for Kubernetes

Add the following block to your values.yml file:

authentik:
    outposts:
        container_image_base: ghcr.io/goauthentik/dev-%(type)s:gh-%(build_hash)s
global:
    image:
        repository: ghcr.io/goauthentik/dev-server
        tag: gh-598e6c8850d2fd1d12f38a54bd93b28c55b491f1

For arm64, use these values:

authentik:
    outposts:
        container_image_base: ghcr.io/goauthentik/dev-%(type)s:gh-%(build_hash)s
global:
    image:
        repository: ghcr.io/goauthentik/dev-server
        tag: gh-598e6c8850d2fd1d12f38a54bd93b28c55b491f1-arm64

Afterwards, run the upgrade commands from the latest release notes.