refactor: generalize fd/ripgrep vars & options

I intend to eventually replace projectile with project.el, so these
doom-projectile-* variables need to be generalized, starting with the
fd/ripgrep executable paths.

ALong with that, this refactors Doom's projectile-generic-command to
lean more on built-in fd support in projectile, where possible (fewer
wheels reinvented).

Ref: doomemacs/core#1

lisp/cli/doctor.el

@@ -219,7 +219,7 @@ in."
                        file (/ size 1024 1024.0))
                 (explain! "Consider deleting it from your system (manually)"))))
 
-          (unless (ignore-errors (executable-find doom-projectile-fd-binary))
+          (unless (ignore-errors (executable-find doom-fd-executable))
             (warn! "Couldn't find the `fd' binary; project file searches will be slightly slower"))
 
           (require 'projectile)