Adds documentation for CVE ID Request button

What does this MR do?

This MR adds documentation for the CVE ID Request button in the issue sidebar. This MR is the documentation portion of Draft: Adds Request CVE ID button to issue sidebar, which was broken into separate merge requests.

For an overview of what purpose the button serves, please see the video below, or read the high-level bullet points below the video:

Slides

• CVE identifiers track specific vulnerabilities in specific version ranges of a project
• GitLab participates in MITRE's CNA program and is able to issue CVE identifiers to projects that are:
  • public
  • hosted on GitLab.com
• Prior to this feature, users needed to manually create an issue to start the CVE request process
• The CVE ID Request button allows project maintainers to directly create the CVE ID Request issue from an existing, confidential issue's sidebar

The CVE ID Request button is only available

• when the project is public
• when the project is hosted on GitLab.com
• when the current user is a maintainer
• when the current issue is a confidential issue

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry Not needed - docs only change
• 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