* docs: make library and README consistent
Generate the bulk of the README from the library documentation, so that
they are consistent using cargo-rdme.
- Removed the Contributors section, as it is redundant with the github
contributors list.
- Removed the info about the other backends and replaced it with a
pointer to the documentation.
- add docsrs example, vhs tape and images that will end up in the README
Fixes: https://github.com/ratatui-org/ratatui/issues/512