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/gc.md

@@ -0,0 +1,76 @@
+# Name
+
+`nix-store --gc` - run garbage collection
+
+# Synopsis
+
+`nix-store` `--gc` [`--print-roots` | `--print-live` | `--print-dead`] [`--max-freed` *bytes*]
+
+# Description
+
+Without additional flags, the operation `--gc` performs a garbage
+collection on the Nix store. That is, all paths in the Nix store not
+reachable via file system references from a set of “roots”, are deleted.
+
+The following suboperations may be specified:
+
+- `--print-roots`
+
+  This operation prints on standard output the set of roots used by
+  the garbage collector.
+
+- `--print-live`
+
+  This operation prints on standard output the set of “live” store
+  paths, which are all the store paths reachable from the roots. Live
+  paths should never be deleted, since that would break consistency —
+  it would become possible that applications are installed that
+  reference things that are no longer present in the store.
+
+- `--print-dead`
+
+  This operation prints out on standard output the set of “dead” store
+  paths, which is just the opposite of the set of live paths: any path
+  in the store that is not live (with respect to the roots) is dead.
+
+By default, all unreachable paths are deleted. The following options
+control what gets deleted and in what order:
+
+- `--max-freed` *bytes*
+
+  Keep deleting paths until at least *bytes* bytes have been deleted,
+  then stop. The argument *bytes* can be followed by the
+  multiplicative suffix `K`, `M`, `G` or `T`, denoting KiB, MiB, GiB
+  or TiB units.
+
+The behaviour of the collector is also influenced by the
+`keep-outputs` and `keep-derivations` settings in the Nix
+configuration file.
+
+By default, the collector prints the total number of freed bytes when it
+finishes (or when it is interrupted). With `--print-dead`, it prints the
+number of bytes that would be freed.
+
+{{#include ./opt-common.md}}
+
+{{#include ../opt-common.md}}
+
+{{#include ../env-common.md}}
+
+# Examples
+
+To delete all unreachable paths, just do:
+
+```console
+$ nix-store --gc
+deleting `/nix/store/kq82idx6g0nyzsp2s14gfsc38npai7lf-cairo-1.0.4.tar.gz.drv'
+...
+8825586 bytes freed (8.42 MiB)
+```
+
+To delete at least 100 MiBs of unreachable paths:
+
+```console
+$ nix-store --gc --max-freed $((100 * 1024 * 1024))
+```
+