docs(plugin): document KUSTOMIZE_PLUGIN_HOME and plugin-home resolution

[alliasgher]

Summary

Adds a "Plugin home" section to `plugin/README.md` describing the directory kustomize uses to look up Exec and Go plugins.

The section documents the four-step resolution order kustomize applies to find the plugin home:

1. `$KUSTOMIZE_PLUGIN_HOME`
2. `$XDG_CONFIG_HOME/kustomize/plugin`
3. `$HOME/.config/kustomize/plugin` (used when `$XDG_CONFIG_HOME` is unset)
4. `$HOME/kustomize/plugin`

It also notes the Windows `$USERPROFILE` fallback for `$HOME` and the relative layout kustomize uses to load an individual plugin (`///`).

Content comes directly from `api/konfig/plugins.go` and `api/internal/plugins/loader/loader.go`.

Fixes #5706.

/kind documentation

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