docs+fix(agent): Update LLM setup instructions & remove GPT-specific flags

[Pwuts]

• Update instructions to set up OpenAI / GPT-4 access
• Add instructions to set up Anthropic access
• Add instructions to set up Groq access
• Remove GPT-specific --gpt3only, --gpt4only CLI flags and related logic
• Remove duplicate config instructions from the docker setup page, replace by a link to the regular setup instructions

Blocks #7091

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 35.01%. Comparing base (4e76768) to head (7942cda).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7183      +/-   ##
==========================================
- Coverage   42.64%   35.01%   -7.64%     
==========================================
  Files          82       18      -64     
  Lines        4772     1211    -3561     
  Branches      655      179     -476     
==========================================
- Hits         2035      424    -1611     
+ Misses       2645      758    -1887     
+ Partials       92       29      -63     

Flag	Coverage Δ
Linux	35.01% <100.00%> (-7.64%)	⬇️
Windows	34.85% <100.00%> (-7.83%)	⬇️
autogpt-agent	35.01% <100.00%> (-0.37%)	⬇️
forge	?
macOS	35.01% <100.00%> (-7.64%)	⬇️

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