Add 'Building configurable components' section to the Frontend Docs

[romaricpascal]

What

Raise a PR to add the drafted content for the new 'Building configurable components' section (internal link) to the 'Building your own JavaScript components' page.

As we'll be bringing content from a Google Doc to plain Markdown, a little care will be necessary around the code blocks (especially around spacing and smart quotes).

Important

This PR will be merged as part of the release process for 5.8.0

Why

So it can be published on the day of the release

Who needs to work on this

Developers

Who needs to review this

Developers

Done when

• A PR is raised to add the 'Building configurable components' section to the 'Building your own JavaScript components' page

[romaricpascal]

Closing as the PR is raised and listed in the tasks of the 5.8.0 release.