Guide on moving a subdirectory package to its own repository

[mtfishman]

This is a summary of the process used in #105961 and #106369 to split off a subdirectory package into its own repository.

[mtfishman]

Following the guide linked to by the last step might be a bit confusing since https://github.com/JuliaRegistries/General#how-do-i-transfer-a-package-to-an-organization-or-another-user also includes the step "Use the [GitHub transfer option][github-transfer] in the settings." but maybe it is clear that isn't needed here.

[ericphanson]

Maybe just write the URL step again?

Manually, in the General edit the repo URL in package's Package.toml file (e.g E/Example/Package.toml), and remove the subdir line.

edit: 'Manually, in the General edit" does not make sense! I guess that is another issue lol

[mtfishman]

I updated the wording in both sections, hopefully that is clearer.

[ericphanson]

LGTM, thanks!