chore: replace concurrently with npm-run-all

[Josh-Cena]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

concurrently pulls in date-fns which is a lot of bloat. The entire dep tree of concurrently is 50MB, compared to npm-run-all's 10MB.

We'll also need to eventually get rid of rxjs in our node_modules. wait-on depends on it, but I believe it's unnecessary. We only have very lightweight usage of wait-on.

This is part of a bigger effort to migrate our building pipeline to something more light-weight while more robust. We'll use run-p extensively for watch, so that CSS editing can be hot-reloaded.

Test Plan

Tested locally by running yarn watch. Hot reloading works.

Test links

Deploy preview: https://deploy-preview-_____--docusaurus-2.netlify.app/

Related issues/PRs

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🔴 49	🟢 100	🟢 100	🟢 100	🟢 90	Report
/docs/installation	🟠 78	🟢 99	🟢 100	🟢 100	🟢 90	Report

[github-actions]

Size Change: 0 B

Total Size: 796 kB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	52.3 kB
website/build/assets/css/styles.********.css	106 kB
website/build/assets/js/main.********.js	599 kB
website/build/index.html	38.9 kB

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