Add prepare_urban_percent to the workflow

[Eddy-JV]

Closes #262 #253 #197 (if applicable).

Changes proposed in this Pull Request

• Integration of new Rule: "Perpare urban_percent", that fetches online yearly dataset from 1950 with predictions until 2050.

• The data is collected from the United Nations CTAD: https://unctadstat.unctad.org/datacentre/

• Filtered for years after 2020.

• Saved in data/urban_percent.csv to be used normally in the workflow.

• New rule in Snakefile is also added.

Checklist

• I tested my contribution locally and it seems to work fine.
• Code and workflow changes are sufficiently documented.
• Newly introduced dependencies are added to envs/environment.yaml and envs/environment.docs.yaml.
• Changes in configuration options are added in all of config.default.yaml, config.tutorial.yaml, and test/config.test1.yaml.
• Changes in configuration options are also documented in doc/configtables/*.csv and line references are adjusted in doc/configuration.rst and doc/tutorial.rst.
• A note for the release notes doc/release_notes.rst is amended in the format of previous release notes, including reference to the requested PR.