Update pr template

[jonavellecuerdo]

Purpose and background context

Clarify language for "Code Reviewer" check list in PR template and update "Environment variables" section in README to make use of code blocks.

• PR template: The updated language relaxes the requirement for Code Reviewers to rerun manual tests that are difficult to execute as long as the Developer provides clear examples (i.e., screenshots, links) that can be verified (e.g., by logging into the AWS consoles and reviewing created outputs).
• README: Reflect our current practices for documenting environment variables in README.

Includes new or updated dependencies?

NO

Changes expectations for external applications?

NO

Developer

• All new ENV is documented in README
• All new ENV has been added to staging and production environments
• All related Jira tickets are linked in commit message(s)
• Stakeholder approval has been confirmed (or is not needed)

Code Reviewer(s)

• The commit message is clear and follows our guidelines (not just this PR message)
• There are appropriate tests covering any new functionality
• The provided documentation is sufficient for understanding any new functionality introduced
• Any manual tests have been performed or provided examples verified
• New dependencies are appropriate or there were no changes