[xsuryanshx]

Summary

• Replaces sphinx-extension-name placeholder in common-issues.md with <extension-name> (angle-bracket syntax) at all 3 occurrences
• Angle-bracket syntax is rejected by pip as invalid, making it unambiguous this is a template placeholder — not a real installable package
• Adds concrete examples (e.g. sphinx-rtd-theme, myst-parser, nbsphinx) to help users understand what to substitute

Closes #114

Test plan

• Verify no remaining occurrences of sphinx-extension-name in the repo
• Confirm the updated examples are clear and instructive

🤖 Generated with Claude Code

[lsetiawan]

Thanks for your contribution, I've integrated this to #118 with other additional fixes across the board.