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 79.30%. Comparing base (c0a240d) to head (f3d8065).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #443 +/- ## ======================================= Coverage 79.30% 79.30% ======================================= Files 22 22 Lines 1822 1822 ======================================= Hits 1445 1445 Misses 377 377 ```

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