Mobile refactoring: adops focus

[bretg]

Some sections that are ready for review:

• [x] iOS GAM Bidding-Only page completely refactored - https://deploy-preview-5507--prebid-docs-preview.netlify.app/prebid-mobile/pbm-api/ios/ios-sdk-integration-gam-original-api
• [x] Android GAM Bidding-Only page completely refactored - https://deploy-preview-5507--prebid-docs-preview.netlify.app/prebid-mobile/pbm-api/android/android-sdk-integration-gam-original-api
• [x] iOS GAM Prebid-Rendered page heavily refactored - https://deploy-preview-5507--prebid-docs-preview.netlify.app/prebid-mobile/modules/rendering/ios-sdk-integration-gam
• [x] Android GAM Prebid-Rendered page heavily refactored - https://deploy-preview-5507--prebid-docs-preview.netlify.app/prebid-mobile/modules/rendering/android-sdk-integration-gam
• [x] iOS Custom Integration page heavily refactored - https://deploy-preview-5507--prebid-docs-preview.netlify.app/prebid-mobile/modules/rendering/ios-sdk-integration-pb
• [x] Android Custom Integration page heavily refactored - https://deploy-preview-5507--prebid-docs-preview.netlify.app/prebid-mobile/modules/rendering/android-sdk-integration-pb
• [x] iOS AdMob integration page heavily refactored (Note to @YuriyVelichkoPI - there's a reference on this page to a closed issue 516) - https://deploy-preview-5507--prebid-docs-preview.netlify.app/prebid-mobile/modules/rendering/ios-sdk-integration-admob
• [x] Android AdMob integration page heavily refactored - https://deploy-preview-5507--prebid-docs-preview.netlify.app/prebid-mobile/modules/rendering/android-sdk-integration-admob
• [x] Android Applovin integration page heavily refactored - https://deploy-preview-5507--prebid-docs-preview.netlify.app/prebid-mobile/modules/rendering/android-sdk-integration-max
• [x] iOS Applovin integration page heavily refactored - https://deploy-preview-5507--prebid-docs-preview.netlify.app/prebid-mobile/modules/rendering/ios-sdk-integration-max
• [x] AdOps Line Item page, new "Big Picture" section - https://deploy-preview-5507--prebid-docs-preview.netlify.app/adops/line-item-creation#the-big-picture
• [x] AdOps Creative page significantly refactored - https://deploy-preview-5507--prebid-docs-preview.netlify.app/adops/creative-considerations
• [x] AdOps AdMob page: added transition, links - https://deploy-preview-5507--prebid-docs-preview.netlify.app/adops/mobile-rendering-admob-line-item-setup
• [x] AdOps Applovin page: added transition, links - https://deploy-preview-5507--prebid-docs-preview.netlify.app/adops/mobile-rendering-max-line-item-setup