[DOC] Branding / build parameter `app.version-number` not documented

[michaelstingl]

Added here:

• [fix/branding refinement] Branding: Color and UI Improvements #1057

Can't find it:

% curl -s "https://doc.owncloud.com/ios-app/next/appendices/mdm.html" | grep "app\.version-number" | wc -l
       0

[michaelstingl]

@hosy make it a build option?

• build.version-number
  (can't be changed dynamically using MDM magic)

Then add it here:

• https://doc.owncloud.com/ios-app/next/appendices/mdm.html#build-options

[michaelstingl]

Appeared here:

• https://github.com/owncloud/ios-app/blob/feature/account-wizard/doc/configuration.adoc