Add pull request template

[ayushdg]

Description

Adds a PR template in the following format.

Usage

N/A

Checklist

• [X] I am familiar with the Contributing Guide.
• [X] New or Existing tests cover these changes.
• [X] The documentation is up to date with these changes.

[ryantwolf]

@ayushdg it looks like there is an awkward amount of extra whitespace between the first checklist item and the rest, can you fix that?