Documentation: Fix Typos, Grammar and Small Crumbs

[manuelroemer]

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• I have read the CONTRIBUTING document
• My code follows the code style of this project
• All new and existing tests passed

Type(s) of Changes

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• Bug fix (non-breaking change which fixes an issue, please reference the issue id)
• New feature (non-breaking change which adds functionality, make sure to open an associated issue first)
• Breaking change (fix or feature that would cause existing functionality to change)
• I have updated the documentation accordingly
• I have added tests to cover my changes

Description

For this PR, I went through most of the .md files and fixed small things (mainly grammar, typos and sentences which were hard to read).

[FlorianRappl]

LGTM - thanks a lot!