Convert HTML files to Markdown files - #4

via Pandoc. The conversion might not be perfect, but that is OK because
these Markdown files are no longer intended to be rendered and published
on the Web.

The conversion is, instead, intended to enable easier comparison of
these files against the files in
https://github.com/EFForg/https-everywhere/tree/master/docs . That will
facilitate closing #2 and
#3 .

(If EFForg/https-everywhere#14423 is accepted,
then the process of comparing the two repositories' documents against
each other will be even easier.)