doc: use backticks for code-in-comments.
auto merge of #11996 : lecram/rust/master, r=pcwalton

This Pull Request aims to add backticks to all instances of code inside comments in snippets present in tutorial.md. It also includes backticks for filenames in the same conditions. See #11796 for motivation.
Move doc/ to src/doc/

We generate documentation into the doc/ directory, so we shouldn't be
intermingling source files with generated files
Remove VPATH usage in Makefiles

This is just messing up all the doc dependencies because of such similar
directory names and file names.

Closes #11422
Rewrite the doc makefile for doc => src/doc

This continues to generate all documentation into doc, but it now looks for
source files in src/doc

Closes #11860
Closes #11970
auto merge of #11891 : alexcrichton/rust/docs, r=brson

I moved all documentation sources (guides, manuals, etc) to `src/doc` to free up `doc` as a build directory for all generated files.

At the same time, I also removed our usage of `VPATH` from the makefiles because it was wreaking havoc with doc deps and it's not very necessary now that we're primarily a rust project rather than having a good portion of C++/C inside of it.