docs(tutor): explain multi-selection yank and paste

[AndroLay]

Closes #11413

Adds a hands-on tutor exercise showing that:

• y stores each selection as a separate register value
• p distributes those values across destination selections in order
• the last value repeats when there are more destinations

The exercise yanks red, green, and blue, creates three numbered destination selections, and produces 1red 2green 3blue.

Validation:

• cargo fmt --all -- --check
• cargo test --workspace -- --test-threads=1
• git diff --check
• completed the new exercise in hx --tutor with default bindings; Helix reported yanked 3 selections to register " and produced the expected line