chore: add new argilla-server folder structure to README.md

[jfcalvo]

Description

In this PR I'm adding a small section to our README.md file with the new folder structure implemented.

@frascuchon I have checked too if it was possible to move /security folder to /api new folder structure but I think it should not be part of /api. In any case it should be moved to /context not sure if it's ok to move it right now as it is. (I have removed one file inside /security that was not used at all)

Refs #4868

Type of change

(Please delete options that are not relevant. Remember to title the PR according to the type of change)

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (change restructuring the codebase without changing functionality)
• Improvement (change adding some improvement to an existing functionality)
• Documentation update

How Has This Been Tested

• Running tests.

Checklist

• I added relevant documentation
• follows the style guidelines of this project
• I did a self-review of my code
• I made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I filled out the contributor form (see text above)
• I have added relevant notes to the CHANGELOG.md file (See https://keepachangelog.com/)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.13%. Comparing base (d1bfec6) to head (9670a24).

Additional details and impacted files

@@               Coverage Diff                @@
##           feat/v2.0.0    #4976       +/-   ##
================================================
+ Coverage        60.92%   92.13%   +31.21%     
================================================
  Files              329      135      -194     
  Lines            17674     5782    -11892     
================================================
- Hits             10767     5327     -5440     
+ Misses            6907      455     -6452     

Flag	Coverage Δ
argilla	?
argilla-server	92.13% <ø> (+0.80%)	⬆️

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