add mgid docs (#1161)
add mgid docs (#1161)

* add mgid docs

* add mgid docs

* add mgid docs

* add mgid docs

* removing setup example

No other bidder doc had an example of defining the pbjs variable, etc. Quite a few have test parameters, so those are cool.

sonobi/prebid.github.io,sonobi/prebid.github.io,sonobi/prebid.github.io,prebid/prebid.github.io,prebid/prebid.github.io,prebid/prebid.github.io