Add an article about jackdaw.admin

[AlexVPopov]

This is a first stab at providing a small guide to the jackdaw.admin namespace.

Checklist

• ~[] tests~
• ~[ ] updated CHANGELOG (the "unreleased" section)~

[codecov[bot]]

Codecov Report

Merging #211 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #211   +/-   ##
=======================================
  Coverage   79.78%   79.78%           
=======================================
  Files          42       42           
  Lines        2572     2572           
  Branches      150      150           
=======================================
  Hits         2052     2052           
  Misses        370      370           
  Partials      150      150

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[AlexVPopov]

@cddr PR updated.