feat(HTML): Fallback showing formulas as images

[dolfim-ibm]

This PR is based on the assumption that item.text should be populated only and only if the formula is coming from a trusted source, i.e. from a declarative backend (like MS Word) or from the code_formula model.

To make this assumption work, the glm_utils.to_docling_document() method will be modified to wipe the item.text and keep only the item.orig.

[mergify]

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