docs: upgrade and simplify tools

[Haroenv]

This PR updates and simplifies the documentation site, so that it can once again run on modern node. Changes are:

• update the bare minimum of metalsmith plugins to run again
• removed unused metalsmith plugins
• removed gulp/webpack/babel (only used for docs watch mode, seems superfluous)
• remove some other un- or barely used packages
• removed watch mode from lint and build (barely used, outdated packages)
• sort package.json dependencies by "usage". This will likely be sorted alphabetically later, but if workspaces would be integrated, they could be split
• add "pr deploys" of the docs to circleci (https://output.circle-artifacts.com/output/job/5510b067-2fb3-4c59-9288-fb222f7172c9/artifacts/0/documentation/reference.html)
• update node version to same one used in InstantSearch (also works with newer ones)

FX-2330

[codesandbox-ci[bot]]

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Latest deployment of this branch, based on commit 165973d0715458226fc7265c961629c9319b8a38:

Sandbox	Source
javascript-helper-app	Configuration
InstantSearch.js	Configuration