Major revisions to documentation.

Major revisions to documentation.
Rewrite VM assignment entirely, adding implementation order and
terminology suggested by Godmar Back.
Make the terminology used elsewhere agree.
Rename "Tour" to "Reference Guide" and move to appendix.
Add "Virtual Addresses" and "Page Tables" sections to reference guide.
Rename "References" to "Bibliography" (to avoid confusion) and move to
end of documentation.
Get rid of option to eagerly load partial pages.
Get rid of fullpage.lds linker script, so rename the normal one from
normal.lds to user.lds.
Break mmu.h into vaddr.h and pte.h.
Add "mcat" to examples as a simpler example of mmap.

NB: Any webpages that reference chapters in the documentation will
need to be updated, because the chapters have been reordered.