Improve documentation

[SabineHaas]

Fix #163, #159

Changes proposed in this pull request:

• Introduce section "local energy system" in Model assumptions in RTD
• Add basic model assumptions to RTD
• Add headings in parameters.rst to make references of these sections possible in RTD
• Improve "scope and limitations" section of RTD with additional information and corrections
• Minor changes in README.rst

The following steps were realized, as well (required):

• Update the CHANGELOG.md
• ❌ Check if full simulation tests pass locally (EXECUTE_TESTS_ON=master pytest)

Also the following steps were realized (if applies):

• Use in-line comments to explain your code
• Write docstrings to your code
• For new functionalities: Explain in readthedocs
• Write test(s) for your new patch of code

Please mark above checkboxes as following:

• Open
• Done

In case of an error due to linting, run black . --exclude docs/ and push your changes.
Note that in case you do not fix a whole issue you should start your PR with Address #xyz.

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

[SabineHaas]

@MaGering I've included some more changes in the same PR so that it's easier to review;)

[MaGering]

Thank you for improving the docs! :) There are a few typos I found during the review. The content looks good to me!

A typo in README.rst in line 74 / 75 that I couldn't correct in review mode. It should be:

You can download the open source [...]

[SabineHaas]

Thank you for improving the docs! :) There are a few typos I found during the review. The content looks good to me!

Thank you @MaGering for correcting the typos and some expressions :)

A typo in README.rst in line 74 / 75 that I couldn't correct in review mode. It should be:

You can download the open source [...]

done with bdafd38