[Documentation]: Feature flags consolidation

[jonniebigodes]

Describe the problem

Related to #31146

With the upcoming release, the documentation will require updates based on the body of work referenced in the associated pull request. This will require the following changes:

• Update the feature flags documentation to reflect the changes introduced in the pull request regarding the essentials addon that was moved into the core package.
• Update the Essentials top-level documentation to factor in the changes introduced
• Vet and test any test-related documentation to ensure that the changes introduced in the pull request are reflected accordingly
• Vet other documentation related to this change to ensure that the changes are reflected accordingly
• Adjust the migration guide to reflect the changes in the feature flags

cc @valentinpalkovic

Additional context

No response