Update internal docs on configuration options

[romaricpascal]

What

Update the repositories' docs about component options to reflect the addition of the ConfigurableComponent. We may be able to point to the Frontend Docs for some of it (however, the part about Nunjucks options is not described in the new 'Building configurable components' section).

Why

So our internal docs are up to date with the code

Who needs to work on this

Developers, Tech writers

Who needs to review this

Developers, Tech writers

Done when

• The 'Component options' docs are up to date

[domoscargin]

While working on this, it strikes me there's some suboptimal overlap between our repo dev docs, and the Frontend Docs site.

It's a bit unclear to me which bits should ideally go where. I personally lean to moving ALL documentation to the site.

I'll make the changes for this PR, but probably something to circle back round to at some point.