Reorganize files

[mark-i-m]

• We should reorganize the .md files in the repo. Currently, a lot of them are at the top-level.
• We should rename the ./part-N.md intro chapters to not have N so that we can move them around.

This might break some links, so maybe it's not worth it?

@rust-lang/wg-rustc-dev-guide What do you think?

[spastorino]

Maybe we can rename files and re-structure things and add redirects?

[jyn514]

We are definitely planning to rename the part-N files, which will also require figuring out how to set up redirects so the links don't break. After that we may or may not move things into subdirectories depending how @igaray feels. Personally I don't think it's particularly important, but it doesn't hurt either.