Make "Configuring ownCloud Web" a partial

[mmattel]

WHAT Needs to be Documented?

We need to make the content of Configuring ownCloud Web a partial and include it at the current location, see
https://doc.owncloud.com/webui/next/owncloud_web/web_with_oC10.html#configuring-owncloud-web

This is necessary, as we are going to have a web_with_ocis section as well and the config content will be the same.

Likely that we need other partials too.

The web_with_ocis section will then be referenced by the web service section in the ocis admin documentation.

WHY Should This Change Be Made?

Flexibility and avoid writing/maintaining stuff twice

@EParzefall