Add AdMob UMP/CMP API code examples & gotchas

[XeduR]

The current documentation pages are incomplete and lack valuable information on how to properly use the new AdMob UMP/CMP API.

This topic has been discussed in detail over at the forums: https://forums.solar2d.com/t/new-requirements-from-google-to-show-ads-cmp/355707. If there are further improvements that someone things could be made, feel free to add to or modify the pages further.

[XeduR]

Colin reminded me on the forums of how appId has not been set via admob.init() in ages, but rather via build.settings, as the main admob documentation page mentions already. I've amended the PR to remove the appId as a parameter in the init function and I've instead added testMode=true to each code example's init call.

[XeduR]

@scottrules44 Could you check if these rewritten and extended AdMob UMP/CMP documentation pages are up to par?

One thought that crossed my mind was adding a mention somewhere that the message needs to also be set up over at AdMob's portal.