By convention, code markup (`~`) is reserved for keybindings in Org-based
documentation in Spacemacs. Verbatim markup (`=`) is reserved for code and
other code-like things. So change several readmes to reflect this convention.
Use verbatim markup for things like (non-exhaustive list):
- Emacs Lisp functions, modes, buffers, etc.
- Environment variables
- Directory paths
- Code in general
- Replace `M-SPC` and `S-M-SPC` by the more conventional `SPC m .`
- Reorganize the code in packages.el
- Add image in README and list bindings with explicit `SPC m` prefix