Fix documentation deployment after repository transfer

[ohno]

Summary

• point Documenter deployment and canonical URLs at JuliaFewBody/TwoBody.jl
• update README and documentation links that still referenced the previous owner
• keep external Antique.jl links unchanged

Root cause

The repository was transferred from ohno/TwoBody.jl to JuliaFewBody/TwoBody.jl, but deploydocs still targeted the old repository. Documenter therefore rejected deployment because GITHUB_REPOSITORY=JuliaFewBody/TwoBody.jl did not match repo=github.com/ohno/TwoBody.jl, while the GitHub Actions job still reported success.

Impact

Documentation builds from main can deploy to the existing gh-pages branch again, and published links now point to https://juliafewbody.github.io/TwoBody.jl/.

Validation

• git diff --check
• verified no stale ohno.github.io/TwoBody.jl or github.com/ohno/TwoBody.jl references remain in README, docs, or workflow files
• local Documenter build not run because Julia is not installed in the local environment; the PR Documentation job provides the runtime validation

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This pull request was developed with assistance from Codex using GPT-5.6 Sol with High reasoning effort.