docs: update buildkit/fsutil fork URLs to EarthBuild org

[gilescope]

Extracted from #442 (the buildkit upgrade) — pure doc-link fix, independent of the bump.

Updates the buildkit/fsutil fork URLs in CONTRIBUTING.md and docs-internals/README.md from the old earthly/* org (and earthly-main branch) to EarthBuild/* (main).

[github-actions]

➖ Are we earthbuild yet?

No change in "earthly" occurrences

📈 Overall Progress

Branch	Total Count
main	5314
This PR	5314
Difference	+0

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Keep up the great work migrating from Earthly to Earthbuild! 🚀

💡 Tips for finding more occurrences

Run locally to see detailed breakdown:

./.github/scripts/count-earthly.sh

Note that the goal is not to reach 0.
There is anticipated to be at least some occurences of earthly in the source code due to backwards compatibility with config files and language constructs.

[gemini-code-assist]

Code Review

This pull request updates repository links to point to the new EarthBuild organization and improves formatting in CONTRIBUTING.md and docs-internals/README.md. The review feedback suggests updating the visible link text in docs-internals/README.md to match the new organization name for consistency, and recommends using a text or empty tag instead of sh for a log output block in CONTRIBUTING.md to prevent incorrect syntax highlighting.

Important

The consumer version of Gemini Code Assist on GitHub is being sunset. Starting June 18, 2026, new organization installations will be blocked, and all code review activity will officially cease on July 17, 2026.
For more details on the timeline and next steps, please review the Help Documentation.