GitBook read-me improvements for installation

[DHaussermann]

We should consider making the following changes to clarify the installation process in the GitBook

A. On server when directing users to managing applications Settings > Apps > Manage Apps Should say Settings > Atlasian Marketplace > Manage Apps.

B. On server install there is a step for: Choose Settings at the bottom of the page, enable development mode, and apply the change. Development mode allows you to install apps from outside of the Atlassian Marketplace.
This is not applicable to server instances and no such change is needed for the installation. Please remove.

C. On Server install the following steps are listed:
1. Upload the OBR file you downloaded earlier.
2. Once the app is installed, press Configure to open the configuration page.

Please add info to step 1. explicitly telling not to add my from URL yet in this modal. Since I've already seen this value earlier it is confusing to me if I should just save it in the field along with the uploaded file.