Docs: a handbook home file is required

[kraftbj]

See #35
See https://wordpress.slack.com/archives/C02RRH4GA/p1740038694447639?thread_ts=1740025944.999619&cid=C02RRH4GA

Resolves a warning:

Warning: A landing page for this handbook has not been created or is not published. You can title it anything you like (suggestions: Scf Handbook or Welcome). However, you must ensure that it has one of the following slugs: welcome, handbook, scf, scf-handbook. The slug will ultimately be omitted from the page’s permalink URL, but will still appear in the permalinks for its sub-pages. Without this page your handbook’s URL will show a seemingly random handbook page.