rustdoc: Add generator meta tag to API docs

This is consistent with the meta tag in Markdown docs run through
rustdoc.
rustdoc: Add rustdoc class to <body> tag

This allows for easier application of user stylesheets to rustdocs no
matter where they are hosted.
auto merge of #16209 : tomjakubowski/rust/rustdoc-html-changes, r=alexcrichton

This PR introduces a couple of minor changes to the rustdoc html templates:

1. The `<meta name="generator" content="rustdoc">` tag now appears in API documentation as well.
2. Adds a `rustdoc` class to the top-level `<body>` tag on all HTML pages.

The second point is a nice-to-have for those who would like to apply a user stylesheet to Rust documentation regardless of where it is hosted. Easier use of user stylesheets may alleviate much of the bikeshedding of personal taste on the rustdoc styles.