Document the integration of an extension to 3rd party app

[petrbroz]

It would be good to document the process of integrating an extension into a 3rd party application, for example, things like:

• copying the extension code and related assets to the right place (images, CSS, etc.)
• loading the extension (via viewer constructor, or using viewer.loadExtension)
• configuring the extension (copy/pasting the "options" object from config.json)

We already had one user run into issues because they copy/pasted the extension files to their project but forgot to configure it based on the config.json file.

[libvarun]

it's explained in https://github.com/libvarun/forge-extensions#understanding-extensionloader-and-using-it-in-forge-app

i guess we should have new section with heading 'Using an extension in your forge app without extension loader'. i did try writing, but step 3 in above section becomes repetitive, we need rearranging. Will think about it and edit.