[TRIVIAL] docs(pull_request_template): add API Impact section

[intelliot]

High Level Overview of Change

Any PR which impacts the API should document those API changes in API-CHANGELOG.md.

That documentation should be added directly, as part of the relevant PR itself. PR authors should write (at least) the initial draft of that API-CHANGELOG entry and push that change to their PR branch so that the documentation change is made alongside the code changes proposed by the PR.

Context of Change

A number of API changes have been made without updating API-CHANGELOG.md.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (non-breaking change that only restructures code)
• Tests (You added tests for code that already exists, or your new feature included in this PR)
• Documentation Updates
• Release

[TRIVIAL]: modifies markdown template only; does not touch any C++ code

[Bronek]

how about adding Chore option above Release (i.e. something that only impacts developer experience e.g. .gitignore, formatting etc.)

[intelliot]

added Chore option above Release (i.e. something that only impacts developer experience e.g. .gitignore, formatting etc.)