Add canonical link to _document.tsx

[elzannewentzel]

Stakeholders: Nick (DG), Fabiana, ROIDNA What: Implement the trailing slashes optimisations specified in the audit feedback. Why: ROIDNA identified inconsistency between internal links pointing to URLs with trailing slashes and resolvingURLs without them.

Assets: Audit document

Requirements:

• Add the rel=canonical in the _document.tsx file