[w3c/payment-request] Editorial: Update links to respec definitions (#891)

Updated all that I can find on respec. I've raised a few PRs to export definitions that payment-request uses. Will update these definitions in separate PRs once they're approved and merged in the other specs.

@marcoscaceres 

PRs to export definitions from other specs
1. [Export iframe `allow` definition](https://github.com/whatwg/html/pull/5193)
2. [Export `language` definition](https://github.com/whatwg/html/pull/5192)
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/payment-request/pull/891

-- Commit Summary --

  * use respec definition for dom event type
  * use respec citation for html body element
  * standardise citation of payment method basic card

-- File Changes --

    M index.html (11)

-- Patch Links --

https://github.com/w3c/payment-request/pull/891.patch
https://github.com/w3c/payment-request/pull/891.diff

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