Document each store type on its own page

This makes for more useful manual table of contents, that displays the
information at a glance.

The `nix help-stores` command is kept as-is, even though it will show up
in the manual with the same information as these pages due to the way it
is written as a "`--help`-style" command. Deciding what to do with that
command is left for a later PR.

This change also lists all store types at the top of the respective overview page.

Co-authored-by: John Ericson <John.Ericson@Obsidian.Systems

doc/manual/utils.nix

@@ -1,5 +1,11 @@
 with builtins;
 
+let
+  lowerChars = stringToCharacters "abcdefghijklmnopqrstuvwxyz";
+  upperChars = stringToCharacters "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
+  stringToCharacters = s: genList (p: substring p 1 s) (stringLength s);
+in
+
 rec {
   splitLines = s: filter (x: !isList x) (split "\n" s);
 
@@ -18,6 +24,8 @@ rec {
     in
       if replaced == string then string else replaceStringsRec from to replaced;
 
+  toLower = replaceStrings upperChars lowerChars;
+
   squash = replaceStringsRec "\n\n\n" "\n\n";
 
   trim = string: