documentation: list pyprojectx as alternative to install PDM

[houbie]

Pyprojectx provides a wrapper script that bootstraps the installation of all the development tools used in a project. As such it has the ability to install PDM on demand.

The benefits:

• you can clone and build a project without having to install PDM first
• you can lock the PDM version: this makes your project immune for breaking changes in PDM; as developer you don't need to install/select the required version
• projects become beginner-friendly as nothing needs to be set up

Pyprojectx has a PDM initializer: it installs PDM locally inside the project and starts pdm init

The px-demo project shows how you can build a PDM project in a minute:

git clone https://github.com/pyprojectx/px-demo.git
cd px-demo
./pw build

IMO it makes sense to add this to the documentation.

[frostming]

Sure, it is a nice tool! Would you like to make the PR?

[houbie]

Will do that