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/source/command-ref/nix-store/serve.md

@@ -0,0 +1,39 @@
+# Name
+
+`nix-store --serve` - serve local Nix store over SSH
+
+# Synopsis
+
+`nix-store` `--serve` [`--write`]
+
+# Description
+
+The operation `--serve` provides access to the Nix store over stdin and
+stdout, and is intended to be used as a means of providing Nix store
+access to a restricted ssh user.
+
+The following flags are available:
+
+- `--write`
+
+  Allow the connected client to request the realization of
+  derivations. In effect, this can be used to make the host act as a
+  remote builder.
+
+{{#include ./opt-common.md}}
+
+{{#include ../opt-common.md}}
+
+{{#include ../env-common.md}}
+
+# Examples
+
+To turn a host into a build server, the `authorized_keys` file can be
+used to provide build access to a given SSH public key:
+
+```console
+$ cat <<EOF >>/root/.ssh/authorized_keys
+command="nice -n20 nix-store --serve --write" ssh-rsa AAAAB3NzaC1yc2EAAAA...
+EOF
+```
+