Link example bug-fix and feature PRs in PR guide

[mvanhorn]

Closes #530

What this PR does

Adds a sub-bullet under each "If you're comfortable writing tests" checkpoint in docs/contributor/pr-guide.md, pointing at a representative merged PR with covering tests:

• The "Fixing an Issue or Bug" tab now links to #1015 (method-resolution fix with new unit and integration tests).
• The "Contributing a New Feature" tab now links to #1198 (force keyword + temporary-filename behavior on download() with new unit and integration tests).

Why these PRs

The issue asks for examples that are small, explainable, and include unit tests. Both linked PRs:

• Touch fewer than 10 files (Fix method resolution bug when base class methods override getattr #1015: 7 files, Download files with temporary names #1198: 6 files)
• Have a single explainable purpose stated in the title
• Add new tests under tests/unit/test_store.py plus integration coverage

Notes

• Per the PR guide's own rule ("docs changes don't need a changelog entry"), no CHANGELOG.md change is included.
• The new sub-bullets are nested under the existing "If you're comfortable writing tests" lines so they sit at the spot the issue called out, without disturbing the surrounding flow or the parallel "if you're not comfortable" guidance below.

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📚 Documentation preview 📚: https://earthaccess--1329.org.readthedocs.build/en/1329/

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