docs: updated nix directory structure

[VirenderNayal]

What kind of change does this PR introduce?

Updates the Nix Directory Structure documentation.

What is the current behavior?

Closes #1957

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

What is the new behavior?

• Added postgres.nix under nix/packages/
• Removed default.nix under nix/ext/