Add new settings in `website` section

[josecelano]

Depends on: torrust/torrust-index#730

Relates to:

• CMS: Terms and demo torrust-index#730
• New static page: Usage Policies and Content Restrictions torrust-index-gui#617

We to update settings type to follow changes in the Index.

[website]
name = "Torrust"

[website.demo]
warning = "..."

[website.terms]

[website.terms.upload]
content_upload_agreement = "..."

[website.terms.page]
title = "Usage Policies and Content Restrictions"
content = "..."

[josecelano]

Closed via #25