docs(local-dev): add pdm install step

[mathisgauthey]

Based on a discussion in #33729, it was missing from the local dev doc.

Changes

I did not find it when trying on my own, I hope it helps future lads in search to contribute to docs.

Context

• Can't build watch documentation locally #33729

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

[renovate-release]

🎉 This PR is included in version 39.127.0 🎉

The release is available on:

• GitHub release
• npm package (@latest dist-tag)
• 39.127.0

Your semantic-release bot 📦🚀

[HonkingGoose]

Should we add the pdm install step to the post-create.sh shell script? This script runs after the GitHub Codespace is created and running.

• .devcontainer/post-create.sh file on main branch
• Line which references the post-create.sh script in the Devcontainer config file on main branch

[mathisgauthey]

Should we add the pdm install step to the post-create.sh shell script? This script runs after the GitHub Codespace is created and running.

* [`.devcontainer/post-create.sh` file on `main` branch](https://github.com/renovatebot/renovate/blob/1e9c00b0b902a36d722596ed42c089fb3220d6ba/.devcontainer/post-create.sh)

* [Line which references the `post-create.sh` script in the Devcontainer config file on `main` branch](https://github.com/renovatebot/renovate/blob/1e9c00b0b902a36d722596ed42c089fb3220d6ba/.devcontainer/devcontainer.json#L29)

I guess it could help as well, yes !