Add beautifulsoup4 dep (used in guide system)

[edan-bainglass]

This should've been added in #763. We haven't had issues because bs4 is a dependency of nbconvert, so we get it from there. However, #1337 updates the installation scheme, which means bs4 is not available when needed (failing tests there). This PR adds the explicit dependency.

Pinging @danielhollas

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.33%. Comparing base (a5b2b92) to head (88278a6).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1465   +/-   ##
=======================================
  Coverage   72.33%   72.33%           
=======================================
  Files         112      112           
  Lines        7245     7245           
=======================================
  Hits         5241     5241           
  Misses       2004     2004           

Flag	Coverage Δ
python-3.11	72.31% <ø> (ø)
python-3.9	72.36% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.