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/architecture/architecture.md

@@ -59,6 +59,7 @@ The [Nix language](../language/index.md) evaluator transforms Nix expressions in
 The command line interface and Nix expressions are what users deal with most.
 
 > **Note**
+>
 > The Nix language itself does not have a notion of *packages* or *configurations*.
 > As far as we are concerned here, the inputs and results of a build plan are just data.