misc: update README screenshots
Summary
On github README.md images aren't correctly rendered.
Furthermore on gitlab image are still stored on a MR branch of sight-doc (ex: https://git.ircad.fr/sight/sight-doc/-/raw/51-enh-doc-update-documentation-to-reflect-sight-21-0-changes/Introduction/media/DicomXplorer02.gif)
Last (but not the least) the intrinsic/extrinsic calibration example shows a big reprojection error, this isn't very attractive.
Relevant logs and/or screenshots
When we click on the missing image
We have an error Content length exceeded
Calibration reprojection:
Possible fixes
Check the size and the path of each screenshot.