Fix lychee link checker false positives for auto-generated Sphinx pages

[Copilot]

The lychee link checker was reporting broken file errors in genindex.html and prf-prf.html because the quantecon-book-theme injects a "Download Notebook" toolbar button linking to /_notebooks/<pagename>.ipynb for every HTML page — including auto-generated Sphinx utility pages that have no corresponding notebook.

Changes

• .github/workflows/linkcheck.yml: Add --exclude-path flags to skip genindex.html and prf-prf.html from lychee's input scan

args: --accept 200,403,503 --exclude-path 'genindex.html' --exclude-path 'prf-prf.html' *.html

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Link Checker Report</issue_title>
<issue_description># Summary

Status	Count
🔍 Total	850
✅ Successful	848
⏳ Timeouts	0
🔀 Redirected	0
👻 Excluded	0
❓ Unknown	0
🚫 Errors	2
⛔ Unsupported	0

Errors per input

Errors in genindex.html

• [ERROR] file:///home/runner/work/continuous_time_mcs/continuous_time_mcs/None | Cannot find file: File not found. Check if file exists and path is correct

Errors in prf-prf.html

• [ERROR] file:///home/runner/work/continuous_time_mcs/continuous_time_mcs/None | Cannot find file: File not found. Check if file exists and path is correct
  Full Github Actions output
  </issue_description>

<agent_instructions>Please review Issue and advise a FIX via PR</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Link Checker Report #173

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