refactor(nix): split flake.nix using flake-parts (#1681)

* refactor(nix): split flake.nix using flake-parts

This commit introduces a modular Nix flake structure using flake-parts.
The monolithic flake.nix has been split into specialized modules.

Here is an overview of the new directory structure:

```
├── flake.nix                  # Root flake file only referencing modules
nix/
  ├── apps.nix                   # Application definitions
  ├── checks.nix                 # Build checks and tests
  ├── config.nix                 # Global configuration
  ├── devShells.nix              # Development environment shell
  ├── fmt.nix                    # Code formatting configuration
  ├── hooks.nix                  # Git hooks and pre-commit
  ├── ext/                       # PostgreSQL extensions
  ├── overlays/                  # Nixpkgs overlays
  ├── packages/                  # Custom packages
  └── postgresql/                # PostgreSQL packages
```

* refactor: move overlays to dedicated directory

The commit moves the overlay configuration from packages/default.nix to a new dedicated overlays/default.nix file for better organization, while keeping the same functionality. This change improves code structure and maintainability.

* refactor(nix): centralize postgresql configuration in config.nix

Moves postgresql version definitions and default settings into a centralized config.nix module.

* refactor(nix): generate postgresql packages into single module

Moves postgresql source and debug package generation into the postgresql module. This reduces duplication by generating packages dynamically based on supported versions.

* refactor: reorganize groonga package and cleanup dependencies

The changes reorganize the groonga-related packages by moving files to a dedicated directory and cleaning up dependencies. It also removes some unused variables and improves package structure.

* feat: configure formatter for nix files

* chore: format nix files

* feat: add git-hooks.nix for pre-commit formatting checks

Adds pre-commit hooks using git-hooks.nix to run treefmt formatting checks before commits.

* docs: generate documentation site with MkDocs

Add MkDocs configuration and assets for generating a documentation site.
This is more readable and provides a structured way to present information about the project.

We don't store the generated site in the repository, but instead use MkDocs to build it on demand.

In a development shell, one can run `mkdocs serve` to quickly preview the site locally.

* fix: add missing recommended direnv

We document the recommended `.envrc` file in the docs, but we forgot to
add it to the repository.

* doc: add documentation about formatting and directory structure

* doc: add helper to generate documentation using mkdocs

mkdocs renders the documentation present in the `nix/docs/` directory in HTML
format.

Author: jfroche

.envrc.recommended

@@ -0,0 +1,3 @@
+watch_file nix/devShells.nix
+
+use flake

.gitignore

@@ -17,6 +17,8 @@ __pycache__/
 result*
 .env-local
 .history
+.envrc
+.direnv
 
 
 #IDE
@@ -25,3 +27,6 @@ result*
 
 db/schema.sql
 common-nix.vars.pkr.hcl
+
+# pre-commit config is managed in nix
+.pre-commit-config.yaml