doc: update license identifier

[daniel-rdt]

Changes proposed in this Pull Request

This PR updates the license identifiers of the files in this repository.

The proposed license Identifier for entirely newly added files is:

# SPDX-FileCopyrightText: Open Energy Transition gGmbH
#
# SPDX-License-Identifier: MIT

For files that already existed where changes and new features were introduced for this project repository the proposed license identifier is:

# SPDX-FileCopyrightText: Open Energy Transition gGmbH and contributors to PyPSA-Eur <https://github.com/pypsa/pypsa-eur>
#
# SPDX-License-Identifier: MIT

This PR follows PyPSA-Eur licensing convention for dotfiles and yaml files to use CC0-1.0 and RST files to use CC-BY-4.0 licensing instead of MIT.

Checklist

• I tested my contribution locally and it works as intended.
• Code and workflow changes are sufficiently documented.
• Changed dependencies are added to envs/environment.yaml.
• Changes in configuration options are added in config/config.default.yaml.
• Changes in configuration options are documented in doc/configtables/*.csv.
• Changes in configuration options are added in config/test/*.yaml.
• Sources of newly added data are documented in doc/data_sources.rst.
• A release note doc/release_notes.rst is added.