[css-view-transitions-1][editorial] More consolidation

[fantasai]

• Move conceptual data held on ViewTransition next to the rest of the conceptual data
• Pull “Monkey Patches” section up since it's part of the module's interface with the rest of the world
• Update / tweak / add headings

This is follow-up to 54cb76d ; it's noted in the commit message but got lost somewhere along the way of interactive rebase. :/

[fantasai]

More specifically, the changes here are:

• Add a subheading for the getters on ViewTransition
• Pull the conceptual data stored on ViewTransition down into the Data Structures section
• Give the various Data Structures subsections headings of the form “Data on ...”
• Pull the monkey patches section up as a direct subsection of the API top-level section, and name it “Integration into Other APIs”

(Marking as a draft because this probably needs to be redone to avoid merge conflicts with other changes.)