Update index.md

[BabaIsYou]

Deleting sentence '→ After pairing a keypad as sensor, it's automatically added to the default alarm system with id "1". '

[manup]

It appears this sentence relates to AncillaryControl added via legacy code:

https://github.com/dresden-elektronik/deconz-rest-plugin/blob/8f82ab547acf50eea0e8c5693b384b477a71d1aa/de_web_plugin.cpp#L7196-L7211

I think this isn't active anymore at least two of the keypads are using DDF.
To streamline legacy keypads (are there any?) above code should be removed.

[devices]$ grep -rn Ancillary

xfinity/THK1_keypad.json:86:      "type": "ZHAAncillaryControl",
frient/kepzb-110_keypad.json:11:         "type":"ZHAAncillaryControl",