doc/contributing/coding-conventions: add a section about file managers

doc/contributing/coding-conventions.chapter.md

···
 
   - `applications/terminal-emulators` (e.g. `alacritty` or `rxvt` or `termite`)
 
+- **If it’s a _file manager_:**
+
+  - `applications/file-managers` (e.g. `mc` or `ranger` or `pcmanfm`)
+
 - **If it’s for _video playback / editing_:**
 
   - `applications/video` (e.g. `vlc`)