Clarify how to add issue numbers in PR template and CONTRIBUTING.md (#8833)

* updating DIRECTORY.md

* Clarify wording in PR template

* Clarify CONTRIBUTING.md wording about adding issue numbers

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Add suggested change from review to CONTRIBUTING.md

Co-authored-by: Christian Clauss <[email protected]>

* Incorporate review edit to CONTRIBUTING.md

Co-authored-by: Christian Clauss <[email protected]>

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Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Christian Clauss <[email protected]>