The documentation of former versions is available in the history (e.g.
by checking out the appropriate git tags), so there is no need to keep
them in parallel to the current documentation.
The document itself uses lambdas later, all the scaffolding to
work around lack of lambdas should be considered for removal, but
that is much larger an effort than I can commit to.
Put occurrences of "#include" in a code span so they are not interpreted
as headers.
Other documents were not broken because the #include was not at the
start of the line, but put them in code spans anyway just in case the
text gets refilled in the future.
