Add docs about tag-based release process

[thekaveman]

This docs page describes in general terms how a release makes its way out via pushing the correctly formatted tags: https://docs.calitp.org/eligibility-server/releases/

Let's add a more step-by-step guide for how this process is accomplished, something that we can follow when we are preparing and publishing a release.

Similar to how the step-by-step guide for Benefits is worded: https://docs.calitp.org/benefits/deployment/release/

Acceptance criteria

• A new heading/section on the above Releases page is created
• This section describes the step-by-step procedure for creating and pushing an appropriate tag for a test and prod release
• This section talks about the automation in place for e.g. creating the GitHub release