Links to guidance site pages do not work on branch deployments

[GCHQ-Developer-741]

Summary of the bug

Explained in this comment, but essentially all internal links to other pages (ones that are written as /page-name in markdown) have the branch name inserted again before the extension, which means they do not work. This does not happen locally or on the main site.

🧐 Expected behaviour

The branch name should not be added twice to internal links