chore(convert): improve HTML Jinja2 template

[jeertmans]

As suggested by @yunusey in #442.

TODO:

• [ ] Fix weird indentation if conditional slide attributes
• [x] Wait before #442 is merged
• [ ] Update HTML example in docs to reflect changes

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 79.96%. Comparing base (e911ec3) to head (426775c). Report is 21 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #443   +/-   ##
=======================================
  Coverage   79.96%   79.96%           
=======================================
  Files          23       23           
  Lines        2011     2011           
=======================================
  Hits         1608     1608           
  Misses        403      403           

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