fix: broken/dead custom renderer links in react-reconciler README.md

[onstash]

Summary

The links for building custom renderers in the react-reconciler README.md file are broken/dead.

This commit replaces them with the Wayback machine snapshot links.

How did you test this change?

1. Clicked on existing links in multiple devices: MacBook Pro M2, MacBook Air M3, iPhone 15 Plus. They were broken/dead.
2. Replaced them with Wayback machine snapshot links. They're now working across all the above devices.

[react-sizebot]

Comparing: 71b3a03...44af8b0

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.js	=	6.68 kB	6.68 kB	+0.05%	1.83 kB	1.83 kB
oss-stable/react-dom/cjs/react-dom-client.production.js	=	605.41 kB	605.41 kB	=	107.21 kB	107.22 kB
oss-experimental/react-dom/cjs/react-dom.production.js	=	6.69 kB	6.69 kB	=	1.83 kB	1.83 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js	=	664.38 kB	664.38 kB	=	117.09 kB	117.09 kB
facebook-www/ReactDOM-prod.classic.js	=	688.25 kB	688.25 kB	=	121.13 kB	121.13 kB
facebook-www/ReactDOM-prod.modern.js	=	678.67 kB	678.67 kB	=	119.48 kB	119.48 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against 44af8b0