Base: Add man page for dirname(1)

Author: https://github.com/tcl3
Commit: 75c61172e3
Pull-request: https://github.com/SerenityOS/serenity/pull/19564
Reviewed-by: https://github.com/gmta ✅

Base/usr/share/man/man1/dirname.md

@@ -0,0 +1,18 @@
+## Name
+
+dirname - return the directory portion of a path
+
+## Synopsis
+
+```sh
+$ dirname [--zero] <path...>
+```
+
+## Options
+
+* `-z`, `--zero`: End each output line with \0, rather than \n
+
+## Arguments
+
+* `path`: Path
+