Correct the project's description of redlines' output

[houfu]

I started this library as a quick way to show differences in text in Streamlit. Since my use case was fairly limited at the time (and this was a side project of a side project), the documentation says that the output of redlines is markdown, since this was the primary method of displaying text in Streamlit.

On a technical note, the output is HTML which can be used in some implementations of Markdown.

Since then, we've also introduced rich output in a TUI.

It's probably a good idea to generalise the description of redlines' output in the docs to fit the current capabilities of the library.