Rename doc/manual{src -> source}

This is needed to avoid this
https://github.com/mesonbuild/meson/issues/13774 when we go back to
making our subproject directory `src`.

doc/manual/src/package-management/ssh-substituter.md

@@ -1,62 +0,0 @@
-# Serving a Nix store via SSH
-
-You can tell Nix to automatically fetch needed binaries from a remote
-Nix store via SSH. For example, the following installs Firefox,
-automatically fetching any store paths in Firefox’s closure if they are
-available on the server `avalon`:
-
-```console
-$ nix-env --install --attr nixpkgs.firefox --substituters ssh://alice@avalon
-```
-
-This works similar to the binary cache substituter that Nix usually
-uses, only using SSH instead of HTTP: if a store path `P` is needed, Nix
-will first check if it’s available in the Nix store on `avalon`. If not,
-it will fall back to using the binary cache substituter, and then to
-building from source.
-
-> **Note**
-> 
-> The SSH substituter currently does not allow you to enter an SSH
-> passphrase interactively. Therefore, you should use `ssh-add` to load
-> the decrypted private key into `ssh-agent`.
-
-You can also copy the closure of some store path, without installing it
-into your profile, e.g.
-
-```console
-$ nix-store --realise /nix/store/m85bxg…-firefox-34.0.5 --substituters
-ssh://alice@avalon
-```
-
-This is essentially equivalent to doing
-
-```console
-$ nix-copy-closure --from alice@avalon
-/nix/store/m85bxg…-firefox-34.0.5
-```
-
-You can use SSH’s *forced command* feature to set up a restricted user
-account for SSH substituter access, allowing read-only access to the
-local Nix store, but nothing more. For example, add the following lines
-to `sshd_config` to restrict the user `nix-ssh`:
-
-    Match User nix-ssh
-      AllowAgentForwarding no
-      AllowTcpForwarding no
-      PermitTTY no
-      PermitTunnel no
-      X11Forwarding no
-      ForceCommand nix-store --serve
-    Match All
-
-On NixOS, you can accomplish the same by adding the following to your
-`configuration.nix`:
-
-```nix
-nix.sshServe.enable = true;
-nix.sshServe.keys = [ "ssh-dss AAAAB3NzaC1k... bob@example.org" ];
-```
-
-where the latter line lists the public keys of users that are allowed to
-connect.