docs: include npm install for site subdirectory

[evans]

- Summary

I got unresolved dependencies errors during the pre-push hooks, since I hadn't installed the dependencies in the site subdirectory. This PR adds instructions to install those missing deps to the contributing guide. If you think there's a better place or alternative, happy to change the PR or you're welcome to take over!

- Test plan

I've tested locally, though I wish there was a way to test/maintain the contributor onboarding experience. If there's something I'm not aware of, I'm down to look at the implementation.

- Description for the changelog

docs: add instructions to install npm dependencies in the site subdirectory

- A picture of a cute animal (not mandatory but encouraged)

༼ つ ◕_◕ ༽つ🤳

[ehmicky]

Thanks @evans for your contribution!

There is a npm run site:build task that could be used instead:

cli/package.json

Line 68 in 060fde2

"site:build": "run-s site:build:*",

[evans]

Thanks @ehmicky!

I thought about adding something similar to the readme's development section, though that seems to be aimed at how to get up and running quickly

[ehmicky]

We just add a discussion with @erezrokah and were thinking npm run site:build:install might actually make more sense, since the linting errors are only due to uninstalled dependencies. What do you think?

[evans]

That makes sense 👍 I'll make the update

Originally I was hesitant to use npm ci when reading into site:build:install. Now I think it's a reasonable decision over npm install, since it'll stay out of the way, installing stable dependencies. I'm assuming there's more value to smoothing the starting path than staying compatibility with the state of the user's site/node_modules, since I'm guessing they're coming to the contributing getting started as a landing page or while debugging. Also a vast majority of contributions are probably made to the main project, so I figure it's a non-issue and changes made to site will be done by someone familiar with the setup.

[ehmicky]

Thanks @evans!