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/binary-cache-substituter.md

@@ -1,50 +0,0 @@
-# Serving a Nix store via HTTP
-
-You can easily share the Nix store of a machine via HTTP. This allows
-other machines to fetch store paths from that machine to speed up
-installations. It uses the same *binary cache* mechanism that Nix
-usually uses to fetch pre-built binaries from <https://cache.nixos.org>.
-
-The daemon that handles binary cache requests via HTTP, `nix-serve`, is
-not part of the Nix distribution, but you can install it from Nixpkgs:
-
-```console
-$ nix-env --install --attr nixpkgs.nix-serve
-```
-
-You can then start the server, listening for HTTP connections on
-whatever port you like:
-
-```console
-$ nix-serve -p 8080
-```
-
-To check whether it works, try the following on the client:
-
-```console
-$ curl http://avalon:8080/nix-cache-info
-```
-
-which should print something like:
-
-    StoreDir: /nix/store
-    WantMassQuery: 1
-    Priority: 30
-
-On the client side, you can tell Nix to use your binary cache using
-`--substituters`, e.g.:
-
-```console
-$ nix-env --install --attr nixpkgs.firefox --substituters http://avalon:8080/
-```
-
-The option `substituters` tells Nix to use this binary cache in
-addition to your default caches, such as <https://cache.nixos.org>.
-Thus, for any path in the closure of Firefox, Nix will first check if
-the path is available on the server `avalon` or another binary caches.
-If not, it will fall back to building from source.
-
-You can also tell Nix to always use your binary cache by adding a line
-to the `nix.conf` configuration file like this:
-
-    substituters = http://avalon:8080/ https://cache.nixos.org/