document the store concept (#9206)

* document the store concept and its purpose

reword the glossary to link to more existing information instead of
repeating it.

move the store documentation to the top of the table of contents, in
front of the Nix language. this will provide a natural place to
document other aspects of the store as well as the various store types.

move the package management section after the Nix language and before
Advanced Topics to follow the pattern to layer more complex concepts on
top of each other.

this structure of the manual will also nudge beginners to learn Nix
bottom-up and hopefully make more likely that they understand underlying
concepts first before delving into complex use cases that may or may not
be easy to implement with what's currently there.

[John adds this note] The sort of beginner who likes to dive straight into reference documentation should prefer this approach. Conversely, the sort of beginner who would prefer the opposite top-down approach of trying to solve problems before they understand everything that is going on is better off reading other tutorial/guide material anyways, and will just "random-access" the reference manual as a last resort. For such random-access the order doesn't matter, so this restructure doesn't make them any worse off.

Co-authored-by: John Ericson <git@JohnEricson.me>

doc/manual/src/SUMMARY.md.in

@@ -16,14 +16,8 @@
   - [Environment Variables](installation/env-variables.md)
   - [Upgrading Nix](installation/upgrading.md)
   - [Uninstalling Nix](installation/uninstall.md)
-- [Package Management](package-management/package-management.md)
-  - [Profiles](package-management/profiles.md)
-  - [Garbage Collection](package-management/garbage-collection.md)
-    - [Garbage Collector Roots](package-management/garbage-collector-roots.md)
-  - [Sharing Packages Between Machines](package-management/sharing-packages.md)
-    - [Serving a Nix store via HTTP](package-management/binary-cache-substituter.md)
-    - [Copying Closures via SSH](package-management/copy-closure.md)
-    - [Serving a Nix store via SSH](package-management/ssh-substituter.md)
+- [Nix Store](store/index.md)
+  - [File System Object](store/file-system-object.md)
 - [Nix Language](language/index.md)
   - [Data Types](language/values.md)
   - [Language Constructs](language/constructs.md)
@@ -35,7 +29,16 @@
     - [Import From Derivation](language/import-from-derivation.md)
   - [Built-in Constants](language/builtin-constants.md)
   - [Built-in Functions](language/builtins.md)
+- [Package Management](package-management/package-management.md)
+  - [Profiles](package-management/profiles.md)
+  - [Garbage Collection](package-management/garbage-collection.md)
+    - [Garbage Collector Roots](package-management/garbage-collector-roots.md)
 - [Advanced Topics](advanced-topics/advanced-topics.md)
+  - [Sharing Packages Between Machines](package-management/sharing-packages.md)
+    - [Serving a Nix store via HTTP](package-management/binary-cache-substituter.md)
+    - [Copying Closures via SSH](package-management/copy-closure.md)
+    - [Serving a Nix store via SSH](package-management/ssh-substituter.md)
+    - [Serving a Nix store via S3](package-management/s3-substituter.md)
   - [Remote Builds](advanced-topics/distributed-builds.md)
   - [Tuning Cores and Jobs](advanced-topics/cores-vs-jobs.md)
   - [Verifying Build Reproducibility](advanced-topics/diff-hook.md)
@@ -97,7 +100,6 @@
     - [Channels](command-ref/files/channels.md)
     - [Default Nix expression](command-ref/files/default-nix-expression.md)
 - [Architecture and Design](architecture/architecture.md)
-  - [File System Object](architecture/file-system-object.md)
 - [Protocols](protocols/protocols.md)
   - [Serving Tarball Flakes](protocols/tarball-fetcher.md)
   - [Derivation "ATerm" file format](protocols/derivation-aterm.md)