Migrate rl-next.md to doc/manual/rl-next directory

doc/manual/rl-next/mounted-ssh-store.md

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+synopsis: Mounted SSH Store
+issues: #7890
+prs: #7912
+description: {
+
+Introduced the store [`mounted-ssh-ng://`](@docroot@/command-ref/new-cli/nix3-help-stores.md).
+This store allows full access to a Nix store on a remote machine and additionally requires that the store be mounted in the local filesystem.
+
+}