Add a docling service file conversion feature

[nerdalert]

• Integrates an automated file conversion via docling-serve to the native knowledge submission component.
• Supports the docling conversion file types of PDF, DOCX, PPTX, XLSX, Images, HTML and AsciiDoc to be used in the next step of context selection (see demo vid below).
• There is a health check of docling-serve at file upload selection, if the service is unavailable, only md files are accepted.

This depends on #439. That PRs changes are in this PR to spare myself another rebase. Will leave this in draft until 439 merges.

Demo .pptx conversion:

pptx-ui-docling-service-demo.mp4

Demo Image conversion:

image-ocr-ui-docling-service-demo.mp4

[Misjohns]

@nerdalert
Does each file get converted into a MD file or do all the files get converted into a single large MD file? My concern is if a user uploads multiple files and we create a single MD for that, it would be more difficult to locate the context in that file. I believe it would be a better UX to create at MD for each uploaded file for the context select step. If needed, we could then merge the MDs together into a single file at submit.

[vishnoianil]

@nerdalert this PR requires rebase.