Add note about incremental rollout to feature flag rollout template

What does this MR do?

Our current rollout procedure does not explicitly call out incremental rollouts.

As a result, we usually test feature flags on our own repos and then enable them globally. That is quite a large blast radius.

In order to contain the risk, we should establish a best practice of performing incremental/gradual rollouts whenever possible.

This MR proposes to add such a note to our feature flag rollout template. If this works well, we can also increase the trust and take load off of the SRE on-call by no longer requiring the on-call to be consulted.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

cc @amyphillips