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-upgrade.md

@@ -0,0 +1,13 @@
+# yadm-upgrade
+
+> Upgrade `yadm` to the latest version.
+> Upgrading will attempt to de-initialize and re-initialize your submodules.
+> More information: <https://github.com/TheLocehiliosan/yadm/blob/master/yadm.md#commands>.
+
+- Upgrade `yadm` to the latest version:
+
+`yadm upgrade`
+
+- Force the upgrade regardless of changes:
+
+`yadm upgrade -f`