Merge developer and user docs into one site

[harigopal]

@pupilfirst/developers This merges the developer and user docs into one site. List of changes:

• Docsify has been removed, and Docusaurus (the latest beta version) is now used to host both the developer and user docs (currently available at docs.pupilfirst.com and developers.pupilfirst.com).
• User docs are accessible at /users, and developer docs at /developers. 🤷🏼‍♂️
• I've removed the docs folder from Lerna/Yarn workspace. I tried quite a bit to get both of them working again, but the forced upgrade to Webpack 5 is super-painful - It'd make more sense to switch away from Webpack altogether for the LMS codebase.

Remaining work

• Update links used in instances of the <HelpIcon> component.

Deploy Checklist

Before merge

• Disable deploys for docs.pupilfirst.com and developers.pupilfirst.com

After merge

• Update one of the web apps to deploy the merged docs.
• Forward developers.pupilfirst.com to docs.pupilfirst.com/developers

Merge Checklist

• Add specs that demonstrate bug / test a new feature.
• Check if route, query, or mutation authorization looks correct.
• Ensure that UI text is kept in I18n files.
• Update developer and product docs, where applicable.
• Prep screenshot or demo video for changelog entry, and attach it to issue.
• Check if new tables or columns that have been added need to be handled in the following services:
• Check if changes in packaged components have been published to npm.
• Add development seeds for new tables.
• If the updates involve Graph mutations ensure that the files are migrated to the new approach without a mutator.

[bodhish]

LGTM

It's missing an after merge checklist.

• Point docs.pupilfirst.com to developers.pupilfirst.com Netlify app
• Add a page rule to forward developers.pupilfirst.com to docs.pupilfirst.com/developers
• Update links in Pupilfirst.com

[harigopal]

This is blocked on #933 - the URLs passed to <HelpIcon> components will need to be moved to en.yml.