rustdoc: clean up docblock table CSS

rustdoc: clean up docblock table CSS

* The rule `display: block` had no noticeable effect. Technically, because
  markdown tables have a tbody and thead, they get wrapped in an [anonymous
  table box] in the CSS tree, nested within the `<table>` element's block
  layout box.

  This rule was added in #87230 to make the table side-scrolling, but
  this same issue was doubly fixed in #88742 by wrapping it in an explicit
  `<div>` tag. Since accessibility advocates recommend the wrapper div over
  marking the table as `display: block`, we'll stick with that.

  https://adrianroselli.com/2020/11/under-engineered-responsive-tables.html

* The rule `width: calc(100% - 2px)` had no visible effect, because the
  anonymous table box was not affected.

* The style is tweaked to basically be the same style GitHub uses.
  In particular, it adds zebra stripes, and removes dotted borders.

[anonymous table box]: https://www.w3.org/TR/CSS2/tables.html#anonymous-boxes