First time user - how to seup BLE message on my MDBT42Q,2v04?

Hi,

can I do this using this device?

Yes, but in Android Google turned automatic scanning for them off by default because it was being abused for advertisements

Effectively what you're asking for is a QR code over Bluetooth?

is there an example code for this? or something to begin with ?

Yep:

• Android: http://www.espruino.com/Eddystone
  
• iOS: https://www.espruino.com/iBeacon
  
  

You basically broadcast a URL/ID and the title comes from the website you link to.

is there any way to know who is connect to my website using the BLE "advertisement"?

You can add a unique ID to the URL - much like how you'd track any other visit on the web.

so I can "save" the mac address ?

NRF.getAddress() will give you the nRF52's address - so you can just add that (or part of it) to the URL.

can I change the text over the internet? i guess I will have to uplaod the all code again ?

Usually the text comes from the website's title, so you can change that easily.

I understand that for 3-4 I need internet connection and a device that can connect to it

Actually I think for what you want, there's a good chance you don't need an internet connection at all.