m4b-tool: add page (#14830)

Co-authored-by: Juri Dispan <juri.dispan@posteo.net>
Co-authored-by: Wiktor Perskawiec <git@spageektti.cc>
Co-authored-by: Lena <126529524+acuteenvy@users.noreply.github.com>

pages/common/m4b-tool.md

@@ -0,0 +1,12 @@
+# m4b-tool
+
+> Merge, split, and manipulate audiobook files with chapters.
+> More information: <https://github.com/sandreas/m4b-tool>.
+
+- Create an audiobook with the audio files in the input directory:
+
+`m4b-tool merge {{path/to/input_directory}} --output-file={{path/to/merged.m4b}}`
+
+- Make chapters using the input files' names:
+
+`m4b-tool merge {{path/to/input_directory}} --output-file={{path/to/merged.m4b}} --use-filenames-as-chapters`