auto merge of #12747 : huonw/rust/rustdoc-markdown, r=alexcrichton

This gives Rustdoc the ability to render our guides, tutorial and manual and modifies the those documents (minor modifications) and makefiles to achieve this.

See commits for more details (especially the makefile rewrite).

Author: bors

mk/clean.mk

@@ -41,7 +41,6 @@ clean-misc:
 	@$(call E, cleaning)
 	$(Q)rm -f $(RUNTIME_OBJS) $(RUNTIME_DEF)
 	$(Q)rm -f $(RUSTLLVM_LIB_OBJS) $(RUSTLLVM_OBJS_OBJS) $(RUSTLLVM_DEF)
-	$(Q)rm -Rf $(DOCS)
 	$(Q)rm -Rf $(GENERATED)
 	$(Q)rm -Rf tmp/*
 	$(Q)rm -Rf rust-stage0-*.tar.bz2 $(PKG_NAME)-*.tar.gz $(PKG_NAME)-*.exe dist

mk/crates.mk

@@ -92,6 +92,7 @@ TOOL_SOURCE_rustc := $(S)src/driver/driver.rs
 ################################################################################
 
 DOC_CRATES := $(filter-out rustc, $(filter-out syntax, $(CRATES)))
+COMPILER_DOC_CRATES := rustc syntax
 
 # This macro creates some simple definitions for each crate being built, just
 # some munging of all of the parameters above.