Bring design in line with playbook refresh

[yndajas]

The accessibility manual was initially largely a clone of the playbook (see #1), copying the styles from circa October 2022. Various playbook PRs have refreshed the design since then, including but probably not limited to:

• Add favicon based on d-marque asset playbook#990
• Replace the d-marque with the full dxw logo playbook#993
• Style the navbar as white on navy playbook#998
• New layout playbook#1204

We should probably bring most of these enhancements into the accessibility manual. We may want to use a different colour scheme (still using the dxw brand colours) for the accessibility manual so that it retains its own identity. This work should be done in collaboration with interaction designers

Note that we shouldn't lose the menu enhancements added to the accessibility manual but not the playbook: dxw/playbook#1337

[yndajas]

We may want to use a different colour scheme (still using the dxw brand colours) for the accessibility manual so that it retains its own identity.

@mingoland suggests we change the top-left corner from navy to blue (from our brand colours): #004876