README.md: Remove nesting modular/shell/

There's no need to hard-code target directories in this repository.
Anyone consuming this material can use submodules and pick their own
target directory.

diff --git a/modular/shell/README.md b/README.md
similarity index 100%
rename from modular/shell/README.md
rename to README.md

diff --git a/modular/shell/cheat-sheet.md b/cheat-sheet.md
similarity index 100%
rename from modular/shell/cheat-sheet.md
rename to cheat-sheet.md

diff --git a/modular/shell/instructor.md b/instructor.md
similarity index 100%
rename from modular/shell/instructor.md
rename to instructor.md