Add English mkdocs

[wm-wm-wm]

Change Summary

Add directory with markdown version of documentation.

Related issue number

Checklist

• Commit message and PR title is comprehensive
• Keep the change as small as possible
• Unit and integration tests for the changes exist
• Tests pass on CI and coverage does not decrease
• Documentation reflects the changes where applicable
• docs/changelog/next_release/<pull request or issue id>.<change type>.rst file added describing change
  (see CONTRIBUTING.rst for details.)
• My PR is ready to review.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.67%. Comparing base (407f593) to head (9d06b75).
Report is 21 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #367      +/-   ##
===========================================
+ Coverage    92.36%   92.67%   +0.30%     
===========================================
  Files          229      229              
  Lines        10245    10247       +2     
  Branches      1021     1021              
===========================================
+ Hits          9463     9496      +33     
+ Misses         587      564      -23     
+ Partials       195      187       -8     

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[dolfinus]

db_connection/ is mostly converted to .md, but is missing in navbar, so these pages cannot be reached

[wm-wm-wm]

commented "nav" section for a while, this switch on auto generated navigation.