DOC-2559: Update key documentation pages with information about the Cloud account creation and the social login option.

[kemister85]

Ticket: DOC-2559

Site: Usage Based Billing Site: Getting Started Site: Cloud Quick Start Site: Introduction to Tinymce Site: Invalid API Key Site: Latest

Changes:

• Add admonition for Cloud account creation and the social login options to various pages.

Pre-checks:

• [x] Branch prefixed with feature/<version>/, hotfix/<version>/, staging/<version>/, or release/<version>/.
• [ ] Update and confirm button links for Google and Github sign up.

Review:

• [ ] Documentation Team Lead has reviewed