Support running maptoposter as a Python tool package callable via uvx #180

Feature Request

Enable the project to be used as a Python tool package, so that it can be installed and run using uvx.

Current Situation

Currently, maptoposter can only be run by cloning the repository and executing it locally on the user's machine. This adds extra steps and complexity for users who just want to quickly run or experiment with the tool.

Proposed Change

Package and expose the main functionality of maptoposter so it can be installed and run using uvx.

Rationale

• Simplifies Installation & Usage: Users will no longer need to clone the repository. Instead, they can run the tool directly with uvx, which will build the script as described in the astral/uv tools guide.
• Better Integration: Running maptoposter as a Python tool package will make it easier to integrate with Python tooling ecosystems and workflows.
• Improved Documentation & Examples: Update docs with clear instructions on using maptoposter via uvx.

Acceptance Criteria

• The project is installable as a tool package (e.g., via pip).
• Users can invoke maptoposter functionality via uvx.
• Documentation is updated with guides and examples for the new installation and usage method.