Closes # (if applicable).

Changes proposed in this Pull Request

Added PLANiT Institute to the users page under the Research Institutes section, including their two PyPSA-based tools: PyPSA_KR (power system analysis toolkit for South Korea) and pyPPA (PPA modeling framework).

Checklist

• [N/A] Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in docs.
• [N/A] Unit tests for new features were added (if applicable).
• [N/A] A note for the release notes docs/release-notes.md of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.