Add only new documents when re-ingesting + add progress bar for ingest process

[janhp]

Dear dev,

Would be great if you could add 2 new features, similar to privateGPT:

1. When I run the ingest process, only new files should be added to the db; already ingested files should be skipped
2. Add a progress bar for the ingest so I know how long I have to wait

[Tchekda]

This feature is implemented in privateGPT and relatively easy to add, I'll make a PR soon to add it. If anyone has some time available before, go ahead