Output RST comment with enough linefeeds.

Empty RST comments need an extra empty line afterwards, re this output
from rst2man:

    (WARNING/2) Explicit markup ends without a blank line; unexpected unindent
Halt rst2man/rst2html on warnings.

We don't allow compiler warnings in the C code, so I don't see any
reason why we should allow warnings in the documentation.

Warnings in the documentation are now build errors.
Delete unreferenced file vcl-intro.rst.

Sphinx complains that this file isn't referenced
anywhere, and a quick glance indicate that the contents
have been merged into other rst files.
Put vmod docs back into the documentation.

Squashing Sphinx warnings revealed that the vmod_std and
vmod_directors includes wasn't working.

Move the generated files a bit so Sphinx is happy to load them.

I'm not using include (where relative paths seem to work)
since that works differently in a TOC listing.
Increase Sphinx strictness.

Warnings are now considered build errors.

We've been shipping documentation with sections missing
because we didn't take warnings seriously.

Bonus change: build in parallel, save 82 centiseconds per build!
(html target on my laptop, ymmv)