Ensure index redirects are relative

[philippjfr]

Fixes #7314

[MarcSkovMadsen]

That will be such a time saver for people like me on a JupyterHub. Thx.

[codecov]

Codecov Report

Attention: Patch coverage is 97.29730% with 1 line in your changes missing coverage. Please review.

Project coverage is 86.60%. Comparing base (50f78eb) to head (18cdbaf).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
panel/io/server.py	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7704      +/-   ##
==========================================
- Coverage   86.60%   86.60%   -0.01%     
==========================================
  Files         346      346              
  Lines       52494    52530      +36     
==========================================
+ Hits        45465    45494      +29     
- Misses       7029     7036       +7     

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[MarcSkovMadsen]

Just tested the main branch on my JupyterHub. Works for me. Thanks.