docs: add animated terminal demo to README

[dobestan]

Summary

Adds an animated terminal demo near the top of the README. The GIF demonstrates:

• Pretty-printing JSON with jq '.'
• Extracting fields with jq '.[].name'
• Filtering with jq '.[] | select(.age > 27)'

Why

The README currently has no visual demo. An animated recording lets first-time visitors immediately see what jq does and how intuitive the syntax is — which is jq's biggest selling point.

The GIF is 145KB and stored in media/.

Recorded with VHS. Demo hosted on AgentGIF.

[wader]

I like the idea. Maybe include the vhs script that produced demo.gif so it can be regenerated/updated?

[itchyny]

The README.md is distributed, but its attached media should be distributed, or not?

[wader]

The README.md is distributed, but its attached media should be distributed, or not?

As in make dist? yes that makes sense. So add to dist_doc_DATA? https://github.com/jqlang/jq/blob/master/Makefile.am#L241