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: #512

Cargo.toml

@@ -155,6 +155,11 @@ name = "demo2"
 required-features = ["crossterm", "widget-calendar"]
 doc-scrape-examples = true
 
+[[example]]
+name = "docsrs"
+required-features = ["crossterm"]
+doc-scrape-examples = false
+
 [[example]]
 name = "gauge"
 required-features = ["crossterm"]