Migrate Runtime and Custom Events docs

[icaraps]

Migrated content:

• https://developer-stage.adobe.com/app-builder/docs/overview/firefly_and_runtime/
• https://developer-stage.adobe.com/app-builder/docs/overview/community/ (Social media and support from Runtime)
• https://developer-stage.adobe.com/app-builder/docs/overview/faq/ (Runtime questions at the end of the page)
• https://developer-stage.adobe.com/app-builder/docs/guides/runtime/
• https://developer-stage.adobe.com/app-builder/docs/guides/events/custom-events/

[icaraps]

Goes together with https://developer-stage.adobe.com/events/docs/ and https://developer-stage.adobe.com/console/docs

[icaraps]

ping @sarahxxu

[rperelma]

@sarahxxu is out this week. @kgera can you review, please?

[purplecabbage]

Can this be merged or closed?

[purplecabbage]

Nag, nag ... this has been sitting around for a long while.
Who can make a final call? @kgera ?

[purplecabbage]

Closing due to inactivity