Add SWML examples to Get Started guides

[hey-august]

Many of our Get Started guides are Compatibility- or cXML- only. We need to update these with SWML and other SignalWire products.

For example: https://developer.signalwire.com/home/calling/voice/getting-started/

Voice

• Get started
  • Making and receiving phone calls (this should be good, but it's a high-traffic doc so worth another review)
  • Forwarding calls
  • Recording calls
  • Setting up voicemail
  • Gathering user input
• Guides
  • Caller ID & CNAM
• SIP stuff
• TTS stuff

Messaging

• Get started
  • Sending your first SMS
  • Receiving your first SMS