Fix ActiveModel, ActiveRecord, ActiveJob README links

The 'Learn More' links in the READMEs for `ActiveModel` and `ActiveRecord` that previously pointed to non-existent `.html` files have been updated to point to the corresponding `.rb` files in the source. In addition, a dead link to the Rails API for `ActiveJob::QueueAdapters` has been replaced with a link to the queue adapters directory in the source and a link to the Edge Rails API for `ActiveJob::QueueAdapters` for convenience.