Add a section of Wallet invocation

[paulbastian]

OpenID4VP has a (small) section on how the Wallet shall be launched/invoked/started. At least copying the section would not seem like a bad idea to me.

Especially section at https://openid.github.io/OpenID4VCI/openid-4-verifiable-credential-issuance-wg-draft.html#section-4.1-1

The Credential Issuer sends Credential Offer using an HTTP GET request or an HTTP redirect to the Wallet's Credential Offer Endpoint defined in Section 11.1.

does not match what many wallets have implemented today. Rather referencing to a Wallet Invocation Section may be a better solution.