[PR Template Update] Updating template per identified dev process improvements

[kelseykropp]

Closes #299

🛠️ Changes Being Made

• Formatting updates & data reorganization
• Added note to document if dev branch should or should not be deleted after merging to main
• New sections and sub-sections added to further detail components of dev work (Data Processing, Testing Dataset, Theiagen Version Release Testing, Reviewer Checklist, and Associated Documentation)
• Modified content of sections (Impacted Workflows/Tasks & Changes Being Made, Workflow/Task Step Changes, Testing, and Final Developer Checklist)

Impacted Workflows/Tasks

Affects Github documenatation of Theiagen PHB PRs

🧠 Context and Rationale

The current PR template is being updated to incorporate process improvements identified during review of the current state of Theiagen's development process.

📋 Workflow/Task Steps

NA

Inputs

NA

Outputs

NA

Impacted Outputs

NA

🧪 Testing

Locally

NA

Terra

NA

Scenarios for Reviewer to Test

NA

🔬 Quality checks

Pull Request (PR) checklist:

• Include a description of what is in this pull request in this message.
• [NA] The workflow/task has been tested locally and on Terra
• [NA] The CI/CD has been adjusted and tests are passing
• [NA] Everything follows the style guide

[sage-wright]

This will be great! Thank you, Kelsey!