Enable invite member modal by default

What does this MR do and why?

• enables the feature flag by default(currently enabled to 100% on .com - https://gitlab.com/gitlab-org/gitlab/-/issues/247208#note_826827946)
• updates the docs per https://docs.gitlab.com/ee/development/documentation/feature_flags.html#document-features-deployed-behind-feature-flags

Related to #344003

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.