docs: remove dead bspwm tutorial link

[leno23]

Fixes #424.

Summary

• Remove the Building a bspwm desktop tutorial entry because its target URL now returns 404.

Verification

• curl -I -L --max-time 20 https://eirenicon.org/knowledge-base/building-a-bspwm-desktop-a-guide/ returns HTTP 404.
• ruby -e "require 'yaml'; YAML.load_file('_data/tutorials.yml'); puts 'tutorials.yml ok'"\n- git diff --check\n- rg -n 'Building a bspwm desktop|eirenicon.org/knowledge-base/building-a-bspwm-desktop-a-guide' _data/tutorials.yml || true\n\nFull Jekyll build was not run locally because this environment is missing the Bundler 2.5.22 version pinned by Gemfile.lock.