Fix #636

Changes proposed in this pull request:

• RTD: Introduce Renewable share of local generation (d099dec)
  ❌ RTD: Introduce Share of demand supplied by renewables (see argument made in Renewable share calculation can be misleading #636)
• RTD: Detail KPI of Renewable factor (d099dec)
• Define Renewable share of local generation in a seperate function
• Define Renewable share with energy carrier weighting
• Adapt benchmark test for both KPI

The following steps were realized, as well (if applies):

• Use in-line comments to explain your code
• Write docstrings to your code (example docstring)
• For new functionalities: Explain in readthedocs
• Write test(s) for your new patch of code (pytests, assertion debug messages)
• Update the CHANGELOG.md
• Apply black (black . --exclude docs/)
• Check if benchmark tests pass locally (EXECUTE_TESTS_ON=master pytest)

Please mark above checkboxes as following:

• Open
• Done

❌ Check not applicable to this PR

_{For more information on how to contribute check the CONTRIBUTING.md.}