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/release-notes/rl-0.7.md

@@ -1,21 +0,0 @@
-# Release 0.7 (2005-01-12)
-
-  - Binary patching. When upgrading components using pre-built binaries
-    (through nix-pull / nix-channel), Nix can automatically download and
-    apply binary patches to already installed components instead of full
-    downloads. Patching is “smart”: if there is a *sequence* of patches
-    to an installed component, Nix will use it. Patches are currently
-    generated automatically between Nixpkgs (pre-)releases.
-
-  - Simplifications to the substitute mechanism.
-
-  - Nix-pull now stores downloaded manifests in
-    `/nix/var/nix/manifests`.
-
-  - Metadata on files in the Nix store is canonicalised after builds:
-    the last-modified timestamp is set to 0 (00:00:00 1/1/1970), the
-    mode is set to 0444 or 0555 (readable and possibly executable by
-    all; setuid/setgid bits are dropped), and the group is set to the
-    default. This ensures that the result of a build and an installation
-    through a substitute is the same; and that timestamp dependencies
-    are revealed.