Intro to Payments

[mDuo13]

There's some tutorial stuff for the Authorized Trust Lines feature, but it's buried in the Authorized Trust lines concept page and in the gateway guide.

These should be split into actual step-by-step tutorials, preferably interactive ones, and updated with newer code samples that don't use rippled in sign-and-submit mode.

[DennisDawson]

This has morphed from an Authorized Trust Lines tutorial (which will still be part of the package) to a higher-level topic that explains trust lines, rippling, and other payment information in a single, middle-layer topic, that includes links to let people drill down on specific areas. I'll accomplish this by describing the three common account types, and how and why each is configured.

[amarantha-k]

@mDuo13 This looks to be done. Please take a look and note if there's more work to be taken care of.