[Platform RFC] Run prettier --write on merge

[GregBrimble]

A couple of assumptions:

1. When making an edit, starting from a consistently formatted page makes creating that edit easier.
2. Not everyone is making these changes in an IDE with Prettier installed (e.g. GitHub online directly)
3. We don't want to block edits which don't conform to formatting rules. We do want to allow them to be made without any additional friction.
4. We don't love reviewing PRs which come from contributors which have an auto-formatter installed on their IDE, so inadvertently pull in unrelated formatting changes with their PRs.
5. Consistent formatting is useful to both contributors, and to our readers (esp. for code snippets).

I propose making a (non-trivial) effort to run prettier --write across the codebase, and then, in order to maintain that consistency going forward, run prettier --write again on every merge atop the contributor's content commits. Couple of ways we could technically do this, and I don't really mind about the specifics. Just wanted to start the conversation here about actually trying to uphold some consistency here.

[Cherry]

Huge +1 to this from me (as per some of my recent PRs), and relates partially to #7798, and pushes way more in that direction.

https://autofix.ci/ is a great modern solution for making this as frictionless as possible for contributors.