Adresses #532

Changes proposed in this pull request:

• Added E-Land_Requirements.rst: Official E-Land requirement list as well as progress on functional and non-functional requirements
• Changed order of readthedocs content

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)