Update JavaScript content in Frontend for v5 changes

[claireashworth]

What

Frontend currently mainly has JavaScript information in 3 pages:

• Import CSS, assets and JavaScript
• Configure components with JavaScript
• JavaScript API Reference

Discuss and decide what content changes are needed and in which pages they should be. Also, discuss the potential for a new page that either brings together the JavaScript info spread across the pages or is specifically about 'Initialising JavaScript'.

Then:

• Draft content changes into agreed locations
• Update cross-links as needed
• Review and publish as part of v5

Why

This is part of the changes to the v5 release

Who needs to know about this

Tech writer, developers

Done when

• Arrange team meeting to discuss JavaScript content updates
• Draft agreed content
• Review and publish with v5

Capturing Colin's action list

• Review every usage of initialising new Component()
  Consider adding a comment about errors that are thrown c1f7a3c
• Add new caveat to “Import JavaScript using a bundler” to mention the downsides of bundling using require()
  This should appear after or within the “bundler that uses CommonJS like Browserify” paragraph
  Caveats such as:
  We ship UMD not CommonJS so no tree shaking is possible.
  When require-ing separate components you’ll see library code duplicated
• Add new heading regarding legacy support for UMD and window globals
  Put the section below “Select and initialise part of a page” heading. Include an example of the Rails asset pipeline
• Add the inline snippet for .govuk-frontend-supported to the “Add the JavaScript file to your HTML” example
  The current example doesn’t include the snippet:

... This change ensures we’re consistent with “Get the JavaScript working” (Get started) and “Check an example page” (Install using precompiled files) HTML examples which both include the snippet already on [the v5 preview](https://deploy-preview-332--govuk-frontend-docs-preview.netlify.app/)

Capturing Romaric's action list

• Writing up whatever we decide the public API of our errors to be
• Adding an issue to update our existing errors to match what we throw during instantiation : Update I18n errors to extend GOVUKFrontendError govuk-frontend#4144

[romaricpascal]

Remaining changes overlap with #373 and #380. We should probably have one edit addressing these 3 things in one go.

[romaricpascal]

Closed by #395 , #397, #398 and #404