docs: add readme for documents directory

2024/documents/README.md

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+# alexandria, the document repository
+
+This is where documents and their derivations are stored. The source code of
+the document and its corresponding `package.nix` are placed in the same place.
+
+There are two ways to organize documents. You can directly place the document
+in [by-name](./by-name/), suitable for documents not assorted with a
+particular course and various miscellany.
+
+For documents related to specific courses, it is preferable to create a
+directory for that course in [by-course](./by-course/), and then nesting the
+document directory within the course directory. This will create a nested
+structure for the packages as well.
+
+For example, the following path:
+
+```
+by-name/my-document
+```
+
+will result in a package set containing `my-document`.
+
+The path
+
+```
+by-course/phil-1/paper-1
+```
+
+results in a nested package set containing `phil-1.paper-1`. That means that
+you can build that document with the command
+
+```sh
+nix build .#phil-1.paper-1
+```
+
+It's evident that this organization structure could be useful for organizing by
+some topics other than courses, so the naming may change in the future.