Add metatype docs

Codecov Report

Merging #680 into master will decrease coverage by 0.01%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #680      +/-   ##
==========================================
- Coverage   86.34%   86.33%   -0.02%     
==========================================
  Files         186      186              
  Lines       13585    13575      -10     
==========================================
- Hits        11730    11720      -10     
  Misses       1855     1855

Impacted Files	Coverage Δ
arkindex/documents/api/elements.py	98.52% <ø> (+0.03%)
arkindex/project/mixins.py	84.61% <0%> (-4.58%)
arkindex/project/urls.py	71.42% <0%> (-1.91%)
arkindex/documents/managers.py	70.14% <0%> (-0.17%)
arkindex/documents/tests/test_element_manager.py	100% <0%> (ø)
arkindex/documents/tests/test_elements_api.py	100% <0%> (ø)
arkindex/project/tests/test_openapi.py	93.47% <0%> (+0.47%)

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added 7 commits

• fec6af72...5084f544 - 6 commits from branch master
• 8c26e701 - Add metatype docs

Compare with previous version

merged

mentioned in commit fc394ce5

mentioned in issue #193 (closed)