docs(editor): add descriptive alt text in freeze-and-origin and groups

[isaganttus]

Continues the alt-text accessibility pass from #805 on two more editor pages.

Changes

editor/fundamentals/freeze-and-origin.mdx — replaced the two origin GIF alt texts ("Origin in the center", "Origin on the left") with descriptions of what the animation actually shows: the rectangle growing outward from its center at 50% X / 50% Y, and growing from the left at 0% X.

editor/fundamentals/groups.mdx — replaced "Groups change Target" and "targets bone visibility" with descriptions of the actions demonstrated: setting a group's Style to Target in the Inspector, and enabling Targets in the View Options menu.

Notes

Alt text follows the same W3C guidance used in #805 — describing the information the image conveys rather than restating nearby body copy.

Verified with mintlify dev. mintlify broken-links reports 33 pre-existing hits across 17 unrelated files, all of them the {{supportForm}} / {{supportFormEducational}} globals from docs.json that the checker doesn't expand — none in the files touched here.