Update the webhook documentation

The actual webhook event looks nothing like in the docs and it differs depending on the other party (i.e. webhook for a movement from a pot would be quite different from webhook for a faster payment).

The code below is a cut from a real working app in Rust and as you can see it is very different from one in the docs:

struct Data {
  id: String,
  created: String,
  description: String,
  amount: i64,
  fees: Fees,
  currency: String,
  merchant: ::serde_json::Value,
  notes: String,
  metadata: Metadata,
  labels: ::serde_json::Value,
  account_balance: i64,
  attachments: ::serde_json::Value,
  international: ::serde_json::Value,
  category: String,
  categories: ::serde_json::Value,
  is_load: bool,
  settled: String,
  local_amount: i64,
  local_currency: String,
  updated: String,
  account_id: String,
  user_id: String,
  counterparty: Counterparty,
  scheme: String,
  dedupe_id: String,
  originator: bool,
  include_in_spending: bool,
  can_be_excluded_from_breakdown: bool,
  can_be_made_subscription: bool,
  can_split_the_bill: bool,
  can_add_to_tab: bool,
  amount_is_pending: bool,
}

struct Fees {}

struct Metadata {
  faster_payment: String,
  fps_fpid: String,
  fps_payment_id: String,
  insertion: String,
  notes: String,
  trn: String,
}

struct Counterparty {
  account_number: String,
  name: String,
  sort_code: String,
  user_id: String,
}

No documentation file or test is named. Start by locating the webhook documentation and comparing its example with the supplied real event shape, including differences between movement sources such as pots and faster payments. Done means the documentation accurately represents the event payloads and their source-specific differences.