fix: replace Imgur-hosted images with self-hosted assets

Imgur withdrew from the UK due to the Online Safety Act, breaking all
images in the docs for UK-based users. This replaces all 195 Imgur
image references across 18 documentation files with locally hosted
copies served from react.dev/images/docs/scientists/.

50 image files are added to cover all direct references and dynamic
size variants (s/m/b/l suffixes used by the getImageUrl helper).

Fixes #8110

Co-Authored-By: Tadao <tadao@travisfixes.com>

Author: travisbreaks