Add additional skip link on guide views

[owenatgov]

What

Adds an additional skip link above the guide contents that goes to the actual content of the guide.

Why

This was identified as a WCAG fail (specifically WCAG SC 2.4.1) as for keyboard only users, it meant that they had the option to skip navigation in the govuk header but still have to tab through the navigation of the guide contents to get to the "actual" content of the guide. This solution of using an additional skip link is used on govuk search pages to skip past filters.

Card: https://trello.com/c/x3QKWP0z/340-cannot-bypass-repeated-navigation

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[injms]

Small visual discrepancy that a bit of margin will sort out - but nothing that stops this from being a solid improvement over not having a skip link at all.