Improve explanation of the different module systems we support

[romaricpascal]

What

• Add some kind of "education" bit before diving into the details: "GOV.UK Frontend supports the following module systems"...
• Clarify that we're still supporting UMD/CommonJS and that migrating to ES Modules, while encouraged can happen in a second stage
• Bearing in mind that people may not be familiar with all these module systems

Why

We've noticed users were not necessarily familiar with which modules systems we were still supporting, or with the terminology around these modules systems

Who needs to work on this

Developers, Tech writers

Who needs to review this

Developers, Tech writers

Done when

• [ ] We've defined how and where we'd clarify this
• [ ] We've update the relevant pieces of documentation

[romaricpascal]

This piece of content should probably go in the "Import CSS, assets and JavaScript" page.

[romaricpascal]

Should be closed when #341 merges

[romaricpascal]

Closed by https://github.com/alphagov/govuk-frontend-docs/pull/395 , https://github.com/alphagov/govuk-frontend-docs/pull/397, https://github.com/alphagov/govuk-frontend-docs/pull/398 and https://github.com/alphagov/govuk-frontend-docs/pull/404