Feature omx docs

[Eunsung-kespion]

Title

docs(omx): add page title and hero image

Summary / Motivation

The OMX doc page started directly with "Order and Assemble the parts," so it lacked a clear page title and visual anchor like other hardware guides. This adds an OMX heading and a hero image hosted on huggingface/documentation-images, consistent with the repo’s guidance to keep large assets out of the git tree.

What changed

• Added # OMX and a top-of-page image (omx_mainimage.png via URL, 70% width) in docs/source/omx.mdx.
• No API or behavior changes; documentation-only.

How was this tested (or how to run locally)

• Optional: doc-builder preview lerobot docs/source/ and open the OMX page to confirm the image loads and layout looks correct.

Checklist (required before merge)

• Linting/formatting run (pre-commit run -a)
• All tests pass locally (pytest)
• Documentation updated
• CI is green
• Community Review: I have reviewed another contributor's open PR and linked it here: #

Reviewer notes

• Confirm the documentation-images asset URL is correct and accessible.
• Image is external URL only (no new binary in the repo).

Please upload this image to https://huggingface.co/datasets/huggingface/documentation-images/resolve/main/lerobot as it is used in docs/source/omx.mdx. Thanks!