The 'grab-wiki' script is unnecessary.

Since April 2012, when the old mlton.org VPS went down for good, there
is no MoinMoin wiki at mlton.org to grab.
Disable 'make-pdf-guide' in 'docs' target of top-level Makefile.

In preparation for importing the AsciiDoc-based guide, disable
building 'mlton-guide.pdf' from the old MoinMoin-based guide.
Remove old MoinMoin-based guide.
Initial import of AsciiDoc-based guide.
Don't include .gitignore and .gitattributes files as attachments.
Use bin/clean for 'make clean'.
Update scripts to use unique prefixes with csplit.
Generate a wiki/Index page.
Organize Makefile.
Introduce ASCIIDOCFLAGS variable to cache conf/asciidoc.flags.
Distinguish source pages from wiki pages.
Condition 'all' target on existence of asciidoc.
Fix location of git-included file in ReturnStatement page.
Fix markup for superscript in MLBasisAnnotations page.
Fix markup/formatting of list in PhantomType page.

Also, a section titled 'References' is automatically treated as a
bibliography by DocBook.
Adjust formatting and markup for Talk-pages.
Adjust formatting and markup of inline literal text in ConcurrentMLImplementation page.
Fix anchor number in Bugs20051202 page.
Fix formatting of link/citation of CejtinEtAl00 in References page.
Add explicit '{empty}' to sections with no content.

Sections with no content yield invalid DocBook XML.