Try more selectors by default to inject the banner

[humitos]

Instead of just using var body = $(config.banner.body_selector); we could try multiple selectors:

    var selectors = ['[role=main]', 'main', 'div.body', 'div.document'];

We should make this backward compatible, allowing just one item for this config.

See this comment https://github.com/readthedocs/readthedocs.org/pull/8079/files#r611035817