docs: simplify concepts section

[arkodg]

• move concepts intro pages one level up
• combine EG resources into the index page

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.83%. Comparing base (c3abe4e) to head (5450dca).
Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6578      +/-   ##
==========================================
- Coverage   70.86%   70.83%   -0.03%     
==========================================
  Files         224      224              
  Lines       38733    38769      +36     
==========================================
+ Hits        27447    27461      +14     
- Misses       9697     9718      +21     
- Partials     1589     1590       +1     

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