Add html output for documenter

[noahrhodes]

closes #443

The addition of Base.showable(m::MIME"text/html", v::VLSpec) = isdefined(Main, :PlutoRunner) in #401 meant that when Documenter checked if html output was supported for VLSpec, pluto wasn't loaded so it did not support creating the interactive examples.

Adding isdefined(Main, :makedocs) is a similar method to add support for the interactive documentation examples.

This fix is needed for the documentation examples in my own package, https://wispo-pop.github.io/PowerPlots.jl/stable/

I would appreciate a quick acceptance and tagging of a new version, as the interactively issue has been an annoyance for over 1.5 years now.

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 78.92%. Comparing base (5e6ad7c) to head (4edeacc). Report is 10 commits behind head on master.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #451   +/-   ##
=======================================
  Coverage   78.92%   78.92%           
=======================================
  Files          12       12           
  Lines         427      427           
=======================================
  Hits          337      337           
  Misses         90       90           

Flag	Coverage Δ
unittests	78.92% <100.00%> (ø)

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