fix: specify DA in onboarding guide title

Purpose:

No title is specified for DA onboarding guide so it comes across as a generic dev guide in the sidebar. This MR adds a title to reduce confusion.

Checks:

Built and run a local copy with docker compose up --build

Navigated to the following URL and confirmed new text appears in the sidebar http://localhost:8000/howtos/

Links https://guidebook.devops.uis.cam.ac.uk/howtos/get-started-working-on-digital-admissions/

Previous MR !384 (merged)