Rollup merge of #25551 - cllns:add-active-class-to-rustbook-toc, r=alexcrichton

Currently the table of contents for `rustbook` doesn't signify which page you are on.

This PR adds an 'active' class to the link for the current page, and defines the CSS rule for that class to make the link underlined and bold.

Not sure about two things:
1) Is `current_page` is a good name for the function parameter? At first I thought `current_item` would be good, but then in the `walk_item` function, you'd have `item` and `current_item`.

2) For the CSS, is both bold and underline too much? At first I had it just be underlined, but that's also how the links look when they're hovered over.

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