Update Nix Directory Structure doc

[kyunghoj]

Improve documentation

Link

https://github.com/supabase/postgres/blob/develop/nix/docs/nix-directory-structure.md#nixext

Describe the problem

The current doc says there exists default.nix file under nix/ext/ and it contains extension registry:

• default.nix - Extension registry and ourExtensions list

but default.nix does not exist in nix/ext/ and ourExtensions list exists in nix/packages/postgres.nix

Describe the improvement

• Add postgres.nix under nix/packages/
• Remove default.nix under nix/ext/

diff --git a/nix/docs/nix-directory-structure.md b/nix/docs/nix-directory-structure.md
index c58441b..5b7d2da 100644
--- a/nix/docs/nix-directory-structure.md
+++ b/nix/docs/nix-directory-structure.md
@@ -107,6 +107,7 @@ Directory containing custom package definitions such as:
   - `start-replica.nix` - Replication tools
   - `migrate-tool.nix` - Migration utilities
   - `dbmate-tool.nix` - Database migration tool
+  - `postgres.nix` - Postgres extension registry and ourExtensions list

 #### `nix/checks.nix`

@@ -132,7 +133,6 @@ PostgreSQL package definitions:

 PostgreSQL extensions:

-- `default.nix` - Extension registry and ourExtensions list
 - Individual `.nix` files - Extension definitions like:
   - `pgvector.nix` - Vector similarity search
   - `pgsodium.nix` - Encryption extension