Add Prettier to contributing documentation

[simonw]

Following #1166 - the docs at https://docs.datasette.io/en/stable/contributing.html should include a section about JavaScript, and it should document how to run Prettier.

I run it in VS Code but it can be run on the command-line too:

npx prettier 'datasette/static/*[!.min].js' --write

[benpickles]

Not sure where exactly to put the actual docs (presumably somewhere in docs/contributing.rst) but I've made a slight change to make it easier to run locally (copying the approach in excalidraw): main...benpickles:prettier-docs

[simonw]

Thanks @benpickles, I just merged that in. I'll use it in the documentation.

# To check code is conformant
npm run prettier -- --check
# To fix it if it isn't
npm run fix

[simonw]

New documentation section here (I documented Black as well): https://docs.datasette.io/en/latest/contributing.html#code-formatting