librustdoc: fix CSS border issue to support Firefox high contrast mode

[robbycbennett]

Because of the thick borders in the Rust documentation result in an unreadable and unpleasant sidebar in Firefox high contrast mode.

Steps to replicate the issue:

1. Open Firefox
2. Enable high contrast mode Settings > General > Website contrast > Custom
3. Go to https://doc.rust-lang.org/std/index.html

This can be fixed by using margin or padding instead of a thick transparent border in src/librustdoc/html/static/css/rustdoc.css

Screenshot:

[robbycbennett]

Here's the workaround, which makes it look the same, although the clickable area isn't as big unfortunately.

.sidebar-crate .logo-container img
{
	border-top: 0;
	margin-top: 0;
}

.sidebar-crate h2 a
{
	border-left: 0;
	margin-left: -0.2rem;
}

.sidebar-elems a, .sidebar > h2 a
{
	border-left: 0;
	margin-left: -0.25rem;
}

[robbycbennett]

Here's a workaround if you have a CSS browser extension. It preserves the same positioning and clickable areas and removes the ugly borders.

.sidebar-crate .logo-container img
{
	border-top: 0;
	padding-top: 16px;
}

.sidebar-crate h2 a
{
	border-left: 0;
	padding-left: calc(0.2rem + var(--sidebar-elems-left-padding))
}

.block ul a
{
	border-left: 0;
	padding-left: calc(1rem + var(--sidebar-elems-left-padding));
}

.sidebar-elems a,
.sidebar > h2 a
{
	border-left: 0;
	padding-left: calc(0.25rem + var(--sidebar-elems-left-padding));
}