docs: use includes for command help output

docs/commands/convert-db.md

@@ -0,0 +1,12 @@
+```
+usage: freqtrade convert-db [-h] [--db-url PATH] [--db-url-from PATH]
+
+options:
+  -h, --help          show this help message and exit
+  --db-url PATH       Override trades database URL, this is useful in custom
+                      deployments (default: `sqlite:///tradesv3.sqlite` for
+                      Live Run mode, `sqlite:///tradesv3.dryrun.sqlite` for
+                      Dry Run).
+  --db-url-from PATH  Source db url to use when migrating a database.
+
+```