GN-98: Update Application Testing document to include entire SDLC

Created by: taylorsperry

Once we have the information required in https://github.com/sourcegraph/sourcegraph/issues/29762, update the SOC2 Application Testing Approach document to describe the SDLC in its entirety:

• How features are spec'd and requirements collected
• How changes are shipped to production (Releases, Cloud)

Definition of Done: The SOC2 Application Testing approach document addresses all of the following:

• The entirety of testing and infrastructure
• The development lifecycle from feature definition through release and deployment
• Links to documentation and resources in other areas of the handbook/docs

Note: The DoD here does not mean an exhaustive list of all technologies that we use. The goal is to have a couple simple, very clear sentences that our audience (internal teammates, auditors, and customers) can understand.