Improve lockfile documentation structure

[tobiasraabe]

Summary

• Restructure lockfile-related how-to documentation around user workflows.
• Rename the portability guide to a move-project guide and update navigation.
• Introduce pytask.lock in the early tutorials and link to follow-up references.
• Add documentation authoring guidance for using Diátaxis.
• Document the change in CHANGELOG.md.

Validation

• uvx --from rust-just just docs
• git diff --check