Refactor code and add tiny features

[MrHedmad]

This pull requests adds the following:

• Refactored some code:
  • Split main.rs into lib.rs and main.rs in preparation for integration tests.
  • Moved some functions around to better reflect their role;
  • Added some docstrings;
  • Removed some useless configurations (like temporary_data_dir from the TOML) and functions;
  • Updated Cargo.lock and removed some useless depenencies.
• Add markdown rendering of pipe descriptions obtained by kerblam run pipe --desc;
• Fix Allow default dockerfile for kerblam run #13 - a default.dockerfile is now detected and used if no specific dockerfile can be found for a pipe. The emoji in the pipes list for such pipes is a smaller fish (:fish:) instead of a whale (:whale:) to differentiate them better.

TODO

Before merging, tick all of these boxes:

• cargo check passes without errors or warnings.
• @all-contributors is made aware of this PR.