Docs: Add reviewer values

What does this MR do?

A MVC for: gitlab-org/frontend/rfcs#86 (closed)

Define a set of Values to deliver maintainer reviews from. This is not a set of rules but rather, a guiding set of principles for maintainers to lean into for consistency in our reviews with one another, much in the same way we have values for delivering code and conduct here in GitLab: https://about.gitlab.com/handbook/values/

Relates to: gitlab-org/frontend/frontend-managers/initiatives#29 (closed)

This work is being broken into two efforts:

Merge	You are here
Add reviewer values	🤘
Add/update reviewer implementation guidelines(breaking down merges, deferring to subject experts, keeping iteration high, acting through collaboration and other proven successful methods to reduce cycle time for reviews)	TBA

Screenshots (strongly suggested)

N/A

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 this is documentation only merge request.
• [-] 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