Enhance documentation, add small fixes and information

[SabineHaas]

Address #154

You can check it out here: https://pvcompare.readthedocs.io/en/docs-enhancing_documentation/index.html

Changes proposed in this pull request:

• Apply new section style
• Minor fixes
• Add some information (minor changes)
• Add sections according to [documentation] Build a comprehensive readthedocs documentation until 20/05/2021 #154

The following steps were realized, as well (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

• ❌ Update the CHANGELOG.md (let's start this after the first release)
  ❌ Apply black (black . --exclude docs/)

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.}