Port user guide over

[selbekk]

This pull request moves all of the user guide data into the docusaurus file structure

Huge PR - but it's mostly copy and pasting, while trying to split the content into manageable groups.

We can discuss the way I've split up the content here - but since this is a huge PR that runs the risk of diverging from what's on master in a matter of days, I hope we can consider re-structuring stuff later on instead.

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The structure is as follows:

Welcome

• About the docs

Getting Started

• Getting started
• Folder structure
• Available scripts
• Supported Browsers
• Supported Language Features

Development

• Syntax highlighting
• In-editor linting
• Debugging in the Editor
• Formatting code
• Proxying in development
• Developing Components in Isolation
• Analyzing bundle size

How-to

• Changing the <title />
• Installing a Dependency
• Importing a Component
• Code Splitting
• Adding a Stylesheet
• Adding a CSS Modules Stylesheet
• Adding a Sass Stylesheet
• Post-Processing CSS
• Adding Images, Fonts, and Files
• Using the public Folder
• Using Global Variables
• Adding Bootstrap
• Adding Flow
• Adding Relay
• Adding a Router
• Adding Custom Environment Variables
• Can I Use Decorators?
• Fetching Data with AJAX Requests
• Integrating with an API Backend
• Using HTTPS in Development
• Generating Dynamic <meta> Tags on the Server
• Pre-Rendering into Static HTML Files
• Injecting Data from the Server into the Page
• Making a Progressive Web App
• Advanced Configuration
• Alternatives to Ejecting

Testing

• Running tests
• Debugging tests

Deployment

• Publishing to npm
• Deployment

Support

• Troubleshooting
• Updating to New Releases
• Feedback
• Something missing?

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Addresses #5238

[amyrlam]

Wow this was a lot of manual work, nice job! It's def tough to review but yeah, I think your approach sounds 💯

[selbekk]

Yeah 3 hours of manual labor :P Should've written a script or something * + *

Anyhow, I'm merging this so we don't diverge too much. :-)