Add N-body gravity simulator series (Go, Programming for Lovers)

[phcompeau]

What is this?

• Title: Build an N-Body Gravity Simulator in Go and Animate the Three-Body Problem
• URL: https://programmingforlovers.com/chapter-4/chapter-4-code-along-go/introduction-to-structs-and-methods-in-go/ (3 parts)
• Section: Go

A three-part Go code-along from Programming for Lovers. Part 1 introduces structs and methods, including pointer receivers. Part 2 organizes the simulation's data types, reads initial systems from file, draws bodies and trails to a canvas, and renders the result as an animated GIF. Part 3 builds a gravity physics engine from Newton's laws (acceleration, velocity, and position updates) and runs the finished program from the command line to animate Jupiter's moons and several periodic three-body orbits. Each part has a video and full code, and the starter code (file I/O and drawing helpers plus data files) is a free download.

Checklist

• This PR adds/fixes exactly one tutorial (or is a docs/tooling-only change).
• Free and open -- no paywall, login wall, or required newsletter signup.
• Project-based -- following it, the reader builds a complete, working artifact.
• I searched README.md for this URL and title -- it is not already listed.
• Entry uses - [Title](https://...) format, placed under the correct section (nested - [Part N](...) for multi-part series).
• No URL shorteners.
• I ran python3 scripts/check_readme.py lint locally and it exits 0.
• Relationship disclosure: I am the author. I (Phillip Compeau) wrote Programming for Lovers.

Lint and check-diff exit 0, and check-links on the three added URLs returns 200 for each.

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