lang/rust: Differentiate doc building bindings
lang/rust: Differentiate doc building bindings

`cargo doc --open` (the previous binding of `SPC m b d`) always opens a
new browser tab. For "doc-driven development" this isn't so nice, since
we'd like to stay in the same tab and just refresh its contents after
editing. That's what `cargo doc` (without `--open`) does, which is
called by `#'rustic-carg-build-doc`.

This commit introduces a binding for the latter behaviour, but asigns it
to the old `SPC m b d`, with the "open" variant as `SPC m b D`. This
matches Spacemacs and follows the usual pattern of "the capital letter
variant is the one you use less often".

hlissner/doom-emacs,hlissner/doom-emacs,hlissner/.emacs.d,hlissner/.emacs.d,hlissner/doom-emacs,hlissner/doom-emacs,hlissner/.emacs.d,hlissner/doom-emacs,hlissner/doom-emacs,hlissner/.emacs.d,hlissner/.emacs.d,hlissner/.emacs.d,hlissner/.emacs.d,hlissner/doom-emacs,hlissner/.emacs.d