Host v4 in its own folder

[romaricpascal]

Cherry picks the commits from the spike exploring how to host docs for v4 in their own folder, next to those of v5.

The commits paint a good pictures of the changes, but as a summary:

• This moves the v4 content in its own folder with v5 content being at the root of the site.
• Links in the content are now relative, to ensure pages for a given version link to other pages of the same version. This applies to both v4 and v5, so it'll be easier to move v5 docs should we need to move it to its own folder in a future version
• Two separate Sass API reference get generated so each version gets the right content for each.
• Pages for the v4 version get their own layout so they can have a separate table of content.
• The header also contains a new menu allowing to navigate between the versions.

Closes #346.

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[36degrees]

I've spent a bit of time trying to get this passing on CI. I've forced push some changes that amend some of the existing commits. They're mainly fixing rubocop violations – you can see the changes here.

I'm not entirely sure about this change, which changes rescue to rescue StandardError to avoid Rubocop's 'Avoid rescuing without specifying an error class' error. I don't know if there's a more specific error class we could be rescuing because I'm unsure what it is we're guarding against.

I've also pushed some additional commits to fix a couple of broken links.

The only other thing I've noticed is that the highlighting of the current page / section in the side navigation is broken in the v4 docs, I think related to some of the logic in the tech docs JavaScript that checks URLs match, but I haven't quite got my head around it yet. I don't think this is critical to shipping v5 but it'd be good to fix if possible.

[romaricpascal]

Cheers for looking into it 😊 That rescue should actually be an ensure (aim is to make the equivalent of a try {} finally {} to ensure things are reset whatever happens), so that'll avoid the rubocop complaints.

[stevenjmesser]

I’m loving the version selector so that users can move between documentation for different versions of GOV.‌UK Frontend. So good!