yadm: add pages (#13920)

Co-authored-by: Sebastiaan Speck <12570668+sebastiaanspeck@users.noreply.github.com>
Co-authored-by: Wiktor Perskawiec <git@spageektti.cc>

pages/common/yadm-perms.md

@@ -0,0 +1,9 @@
+# yadm-perms
+
+> Update permissions.
+> It is usually unnecessary to run this command, as `yadm` automatically processes permissions by default. This automatic behavior can be disabled by setting the configuration `yadm.auto-perms` to `"false"`.
+> More information: <https://github.com/TheLocehiliosan/yadm/blob/master/yadm.md#permissions>.
+
+- Change file permissions:
+
+`yadm perms`