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

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doc/contributing/coding-conventions.chapter.md
··· 338 339 - `applications/terminal-emulators` (e.g. `alacritty` or `rxvt` or `termite`) 340 341 - **If it’s for _video playback / editing_:** 342 343 - `applications/video` (e.g. `vlc`)
··· 338 339 - `applications/terminal-emulators` (e.g. `alacritty` or `rxvt` or `termite`) 340 341 + - **If it’s a _file manager_:** 342 + 343 + - `applications/file-managers` (e.g. `mc` or `ranger` or `pcmanfm`) 344 + 345 - **If it’s for _video playback / editing_:** 346 347 - `applications/video` (e.g. `vlc`)