Fix all grammar mistakes in the documentation

[zaleslaw]

[Jolanrensen]

While a part of the grammar of the documentation is fixed, all documentation on the website needs to be checked and overhauled. The priority is to have docs for everything, even if they aren't perfect, but in some cases the meaning of the docs becomes vague because of grammar errors.
These fixes can be done in several PRs or in one big one and need to entail:

• grammar checks (missing articles, erroneously used words)
• spelling checks (mostly okay)
• consistency between data frame, DataFrame, and the interface DataFrame (or DataFrame instance)
• fixes inside code examples (with Korro)
• have consistent Title case Usage in headers (Let's start with a capital and drop the rest)