This change is needed because in 1.8.3 we are going to introduce Markdown tables
in the documentation (#3873 and #3921), and the old marked version did not
support generating them.
Instead of committing the marked source code here, we live install from npm if
needed via the Makefile.
n.b.: at the time of this change, marked latest version is 1.0.0, released a few
days ago. I am updating to the version immediately before that (0.8.2),
because in 1.0.0 the hyperlinks in the Table of Contents do not work
(probably a bug in that version).
