Apache configuration for Let's Encrypt webroot validation

[nztim]

This PR originates from #9366. It might be helpful to include something in the docs to make it as easy as possible to implement SSL.

In this PR I simply added a note but you could alternatively add the line to the actual config block with a comment, have a separate section/page going into more detail, or achieve it another way.

[codecov-io]

Codecov Report

Merging #9438 into master will decrease coverage by <.01%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #9438      +/-   ##
==========================================
- Coverage    41.5%   41.49%   -0.01%     
==========================================
  Files         570      570              
  Lines       74324    74328       +4     
==========================================
- Hits        30847    30844       -3     
- Misses      39636    39642       +6     
- Partials     3841     3842       +1

Impacted Files	Coverage Δ
routers/init.go	67.81% <50%> (-0.86%)	⬇️
modules/log/event.go	64.61% <0%> (-1.03%)	⬇️
models/pull.go	52.91% <0%> (-0.65%)	⬇️

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