add schema documentation

[pgoellnitz]

globe-swiss/phaenonet-client#648

• migrate schema documentation from dbml to mermaid
• add statistics collection
• update readme

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.79%. Comparing base (b781441) to head (8ee0e9f).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #314   +/-   ##
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  Coverage   94.79%   94.79%           
=======================================
  Files          33       33           
  Lines        1632     1632           
  Branches      150      150           
=======================================
  Hits         1547     1547           
  Misses         71       71           
  Partials       14       14           

Flag	Coverage Δ
unittests	94.79% <ø> (ø)

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