Docs: note the -800 HU floor in the density conversion

[danielemolino]

Follows #415. Documentation only, no behaviour change, consistent with #378 being closed as wontfix for DiffDRR.

Two additions:

• a docstring on transform_hu_to_density describing what the thresholds do, including
  that everything at or below −800 HU is collapsed onto a single value;
• a callout in the read docs noting that this removes most aerated lung parenchyma on
  thoracic CT, and pointing users who need it preserved to nanodrr.hu_to_mu.

The intent is only to make the convention visible to someone loading a chest CT, since
transform_hu_to_density is exporti and the behaviour isn't discoverable from the docs
today.
Happy to reword or trim anything, including dropping the nanodrr pointer if you'd rather not cross-reference packages here.

[github-actions]

Thank you for your PR! Please ensure you've read DiffDRR's development guide.

[eigenvivek]

@danielemolino thank you for the PR! can you please fix the failing test (should just need to run nbdev_clean; nbdev_export), then happy to merge

[eigenvivek]

thanks, now you can see why i moved away from nbdev for future projects :)

closes #416

[danielemolino]

Haha, I can definitely see why, thanks for the review and for merging!
At least this gave me the chance to learn a few new commands along the way