website/integrations: gitlab: add oidc guide

[motey]

Details

Adding a guide to connect GitLab via OIDC (alternative to SAML which is the only guide at the moment)

Discussed in:

Closes #8175

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Checklist

• Local tests pass (ak test authentik/) <- not applicable; only doc changes
• The code has been formatted (make lint-fix) <- not applicable; only doc changes

If an API change has been made

• The API schema has been updated (make gen-build) <- not applicable; only doc changes

If changes to the frontend have been made

• The code has been formatted (make web) <- not applicable; only doc changes

If applicable

• The documentation has been updated
• The documentation has been formatted (make website) <- Yes, but very unsure about this. should that not happen on "server"-side (github actions) in a controlled environment. Needed to install a lot of stuff on my machine for that :D not very contributor friendly

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 49.73%. Comparing base (9dc813d) to head (de2d1df).
Report is 26 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #9740       +/-   ##
===========================================
- Coverage   92.39%   49.73%   -42.66%     
===========================================
  Files         704      702        -2     
  Lines       34403    34162      -241     
===========================================
- Hits        31785    16990    -14795     
- Misses       2618    17172    +14554     

Flag	Coverage Δ
e2e	48.32% <ø> (-1.34%)	⬇️
integration	25.36% <ø> (ø)
unit	?

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