[Haddock] Add grey background to inline code

In line with what other programming languages show, monospaced text will now have a slightly grey background:

as without it, specially non-linked monospaced words were very difficult to spot:

This follows the same style as Rust does, which in my opinion looks very readable

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This is probably a matter of preference and esthetics so I think it would be perfectly understandable if the PR is not accepted.