Change documentation URL (#8898)

app/javascript/mastodon/features/getting_started/index.js

...cript/mastodon/features/ui/components/onboarding_modal.js

app/views/home/index.html.haml

app/views/layouts/public.html.haml

config/locales/ar.yml

config/locales/ca.yml

config/locales/co.yml

config/locales/cs.yml

config/locales/cy.yml

config/locales/da.yml

config/locales/de.yml

config/locales/el.yml

config/locales/en.yml

config/locales/eo.yml

config/locales/es.yml

config/locales/eu.yml

config/locales/fa.yml

config/locales/fi.yml

config/locales/fr.yml

config/locales/gl.yml