feat: introduce ODMClient for simplified multi-database management

[A-K-6]

Problem Statement

Managing multiple MongoDB databases and their associated Beanie models often requires significant boilerplate code to handle connections, migrations, and initialization for each database individually.

Solution: ODMClient

This PR introduces the ODMClient, a high-level utility class that centralizes and simplifies the lifecycle of Beanie applications.

Key Features

• Batch Initialization: Use init_db with a configuration dictionary to initialize multiple databases and models in one call.
• Centralized Connection Management: Uses AsyncMongoClient internally and supports async context manager usage (async with ODMClient(...) as client:).
• Integrated Migrations: Automatically handles migrations during database registration with a built-in lock for safety.
• Full Feature Support: Passes through all standard Beanie init parameters (allow_index_dropping, recreate_views, skip_indexes).
• Improved Documentation: Includes a new tutorial and automated API documentation generation.

Examples

from beanie import ODMClient, Document

class User(Document): ...
class Product(Document): ...

db_config = {
    "user_db": [User],
    "product_db": [Product]
}

async with ODMClient("mongodb://localhost:27017") as client:
    await client.init_db(db_config, migrations_path="path/to/migrations")

Checklist

• Code implementation of ODMClient (beanie/odm/client.py)
• Comprehensive tests in tests/odm/test_client.py
• Documentation added in docs/odm-client.md
• mkdocs.yml / pydoc-markdown.yml updated for navigation
• Tests for ODMClient passing locally