Custom 404 page

[gjong]

Labels: enhancement, priority:P2
Parent issue: Epic: Documentation site overhaul

Problem
The default 404 page has no links back into the docs.

Suggested fix

• Add a custom 404 layout with links to Getting Started and the search modal (depends on Importing profile with archived contract or accounts causes issues #7).

Acceptance criteria

• Custom 404 page live with working navigation links.