The book was located under 'src/doc/trpl' because originally, it was
going to be hosted under that URL. Late in the game, before 1.0, we
decided that /book was a better one, so we changed the output, but
not the input. This causes confusion for no good reason. So we'll change
the source directory to look like the output directory, like for every
other thing in src/doc.
This adds strictly more information to the source files and reduces the
need for customized tooling to render the book.
(While this should not change the output of _rustbook_, it is very
useful when rendering the sources with external tools like Pandoc.)
