Add Rust snippets for columnar updates of custom data

[teh-cmc]

And clean up the Feature section of the snippet index while I'm here.

[github-actions]

Web viewer failed to build.

Result	Commit	Link	Manifest
❌		https://rerun.io/viewer/pr/8778	+nightly +main

^{Note: This comment is updated whenever you push a commit.}

[teh-cmc]

@rerun-bot full-check

[github-actions]

Started a full build: https://github.com/rerun-io/rerun/actions/runs/12911344875

[teh-cmc]

@rerun-bot full-check

[github-actions]

Started a full build: https://github.com/rerun-io/rerun/actions/runs/12911608996

[teh-cmc]

@rerun-bot full-check

[github-actions]

Started a full build: https://github.com/rerun-io/rerun/actions/runs/12913520582