Enable alternative text (alt) support for images in Docs and exports

[Ovgodd]

Is your feature request related to a problem? Please describe.
It is currently not possible to define alt text for images imported into a blocknote. This limits accessibility

Describe the solution you'd like
Allow users to define alternative text for each imported image. If no text is provided, the exported HTML or document should contain an empty alt attribute like: .
This alt text should also be included when exporting to Word documents (ODT/DOCX), so users don't have to re-enter it.

Describe alternatives you've considered
Leaving the alt attribute empty by default or editing alt text manually in exported files, but these options reduce accessibility.

Additional context
This feature would improve document accessibility and align with best practices for semantic content and screen readers.

[cyberbaloo]

It would also be exported to PDF documents if there is an alternative text or a textual alternative if the image is informative.

[woutervroege]

Linking Product Goal: TypeCellOS/Product#16