Media Guide – Add missing info for pre-recorded video with audio

[remibetin]

Re-creates @selfthinker's https://github.com/w3c/wai-media-guide/pull/201

Direct link to preview: https://deploy-preview-584--wai-website.netlify.app/media/av/planning/

Initial description

The table at the bottom of the 'Planning Audio and Video Media' page that summarises what WCAG says about time-based media is missing two things which this PR adds:

1.2.3 is already mentioned under audio description that it's also valid for transcripts. But it is missing from the transcript column. 1.2.7 is missing information about extended audio descriptions. Not sure if it was potentially left out intentionally to save space?