Update docs site

[regadas]

• Upgrade sbt-microsites to 1.3.1
• Migrate from tut to mdoc

[codecov-io]

Codecov Report

Merging #920 (6d6fe73) into develop (ff8b0e6) will increase coverage by 0.07%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #920      +/-   ##
===========================================
+ Coverage    89.67%   89.74%   +0.07%     
===========================================
  Files          119      119              
  Lines         9869     9869              
  Branches       494      494              
===========================================
+ Hits          8850     8857       +7     
+ Misses        1019     1012       -7     

Impacted Files	Coverage Δ
...main/scala/com/twitter/algebird/monad/Reader.scala	52.94% <0.00%> (-5.89%)	⬇️
.../scala/com/twitter/algebird/immutable/BitSet.scala	96.35% <0.00%> (-0.43%)	⬇️
.../main/scala/com/twitter/algebird/BloomFilter.scala	95.13% <0.00%> (+1.32%)	⬆️
...src/main/scala/com/twitter/algebird/Interval.scala	86.95% <0.00%> (+6.08%)	⬆️

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[johnynek]

Thanks for taking this!