Update documentation with architectural and development patterns to prevent transient bugs

What does this MR do?

Updates our documentation with architectural and development patterns that can prevent transient bugs from occurring.

Also includes manual testing methodology to improve likelihood of being caught during development.

Tasks

• Add some content for Frontend
• Invite Frontend engineers to add content
• Add some content for Backend
• Invite Backend engineers to add content

Note: If the volume of content justifies it, we can split each FE and BE into separate pages. Starting simple, for now.

Screenshots (strongly suggested)

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

Resolves #293858 (closed)