FIX - Crash in translator when using a non HTML builder

[s-weigand]

This change skips the transpator setup if the builder is not targeting html as the format.

In our readthedocs config, we use formats: all which besides HTML also builds PDF using the latex builder, causing the following error:

Exception occurred:
  File "/home/docs/checkouts/readthedocs.org/user_builds/pyglotaran-extras/conda/latest/lib/python3.10/site-packages/pydata_sphinx_theme/translator.py", line 44, in visit_table
    self._table_row_indices.append(0)
AttributeError: 'BootstrapHTML5Translator' object has no attribute '_table_row_indices'

This builder has the default_translator_class defined, but it is not compatible with the BootstrapHTML5TranslatorMixin and thus causes the AttributeError above.

This can be prevented by first checking if the builder uses the html format, which is true for the html and singlehtml builders.

Since the builder format does not change and is independent of the translator, it makes sense to lift it out of the loop over translators.

[s-weigand]

Sorry I didn't look at the other open PRs and it turns out this is duplicate of #2076