fix: Missing button text for PostScript

[evyncke]

This fixes #6900 by avoiding any Postscript file in the list of built URL for a document,

[evyncke]

Note: I was unable to fully test it as my docker environment does not have the full archived drafts, but add empty draft-ietf-tsvwg-quickstart-07.ps/txt/pdf locally and it works (i.e., txt/pdf buttons are displayed and nothing for ps)

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 88.94%. Comparing base (187c2c5) to head (b31e446).
Report is 64 commits behind head on main.

❗ Current head b31e446 differs from pull request most recent head 4478125. Consider uploading reports for the commit 4478125 to get more accurate results

Files	Patch %	Lines
ietf/doc/utils.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7200      +/-   ##
==========================================
- Coverage   88.98%   88.94%   -0.05%     
==========================================
  Files         291      291              
  Lines       40717    41069     +352     
==========================================
+ Hits        36233    36528     +295     
- Misses       4484     4541      +57     

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