Improving the SD-JWT examples

[David-Chadwick]

The current examples of SD-JWT in section 3.2, whilst correct, only comprise the credential with the SD fields flagged (using !sd) prior to securing, followed by the SD-JWT secured verifiable credential. It may be helpful to the reader to show the contents of the unencoded JWT body, viz:

{
  "_sd_alg": "sha-256",
  "@context": [
    "https://www.w3.org/ns/credentials/v2",
    "https://www.w3.org/ns/credentials/examples/v2"
  ],
  "issuer": "https://university.example/issuers/565049",
  "validFrom": "2010-01-01T19:23:24Z",
  "credentialSchema": {
    "_sd": [
      "C7P4_rPSwJJ63kDm6K7PI4E39IXCDu_a98tmI-8UlwA",
      "fechAzukKKvWk_c3JfU-Sk_GVcX3CFGeuJfN6MNL_uQ"
    ]
  },
  "credentialSubject": {
    "degree": {
      "name": "Bachelor of Science and Arts",
      "_sd": [
        "wy-UoUQJmXJndaMROqTapQGJ8EUU8Dz8LwtCO0g3gnU"
      ]
    },
    "_sd": [
      "MCvCBoKRoUiivsPEfg00nAuGr8DwaWUeAkHuBNq4zdI"
    ]
  },
  "_sd": [
    "2D5lkIK2hrAd2a4QK_dtoJI8ZEOgKYaXKhOngaxvRsc",
    "WTfYOzSQhejkIqe5IYDkNJrqw5r4KpxHgOaaV04hsMQ"
  ]
}

and add some description of the SD-JWT such as: "the six fields preceded by a tilde that follow the JWT signature are base64 encodings of the six selectively disclosed properties"

[decentralgabe]

At first this was avoided for verbosity sake, though I can see the benefit in adding it.

[TallTed]

A rephrased description of the SD-JWT --
the tilde and subsequent six fields that follow the JWT signature are base64 encodings of the six selectively disclosed properties

[bc-pi]

from #270 (comment):

... and defer more significant changes to #264. And also defer addressing other things like the the entire concept of securing a VP with SD-JWT being nonsensical and/or other issues that maybe/hopefully will be surfaced by doing that work.