Use create/delete words in self-monitoring docs

What does this MR do?

As suggested in https://gitlab.slack.com/archives/C0NFPSFA8/p1582903339143800?thread_ts=1582865083.127300&cid=C0NFPSFA8:

Under https://docs.gitlab.com/ee/administration/monitoring/gitlab_self_monitoring_project/#deactivating-the-self-monitoring-project, there’s a big warning about deactivating. It sounds like it’s not possible to reactivate once deactivated. Why not? Can’t I just delete the project and then add it again following the instructions above? And why is it called “deactivate” instead of “delete” which is what it seems to actually do?

Activating/Deactivating can be confusing to users. This MR changes the terms to create/delete instead, and makes the warning clearer.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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