Add docs for scheduled export of contacts

[RCheesley]

It turns out nobody ever wrote the docs for https://github.com/mautic/mautic/pull/11272 which has resulted in some confusion (see https://forum.mautic.org/t/bulk-contact-export-email-does-not-arrive/31169/).

This PR adds the missing documentation in the configuration section (to turn on/off the background sending of exports) and also for the command line syntax/cron job information.

[mautibot]

This pull request has been mentioned on Mautic Forums. There might be relevant details there:

https://forum.mautic.org/t/bulk-contact-export-email-does-not-arrive/31169/17

[RCheesley]

Thanks for the review @uadf 🚀

[RCheesley]

@all-contributors please add @uadf for review

[allcontributors[bot]]

@RCheesley

I've put up a pull request to add @uadf! :tada: