Client extensions - post-release docs improvements part 1

[andrew-walford-prisma]

There are a few outstanding tasks to resolve that came up in the docs reviews, but we ran out of time to do.

I've tagged this as 4.8.0, but we can merge these changes - when they're done - at any time.

SMEs: Pierre, Serhii

• Add links to the extensions docs from Middlewares doc. Also, from any others?
• - Add description of 'Prisma.getExtensionContext'. See here.
• We need a note in the docs to cover the case where if you call query twice in a row, it executes the query twice. See here. The example, here, I think is misleading. Pierre has opened a PR to fix this specific issue, so I'm ticking this one off.