docs(css): optimize sponsor-section background style

[imfenghuang]

Description

When the adblock plugin is opened in chrome, the https://sponsors.vuejs.org/vite.json is net::ERR_BLOCKED_BY_CLIENT and then the sponsor-grid is empty, so that the svg with class bglines is above the button text, because of the position: absolute and z-index: 0.

So, i changed the styles, use background to instead of inserting svg tag directly.