Improvements to "how to use Sigstore" on main Overview page

[smythp]

The links under the "How to use Sigstore" heading on the main Overview page are arguably the main starting place for those new to the project. Currently, the first link simply says "quickstart," and links to a page about getting started with Cosign.

I think there's some room for improvement here, with more comprehensible language in the calls to action, and we might also link some of the help and contributing pages.

If we're feeling ambitious, we might also change the design a bit and use icons, logos, or fancier color. (I'm somewhat inspired here by affordances provided by the Hugo replatform test site.)''

Arose in discussion on #210.

[smythp]

I'm closing this after the changes to the intro by @jonvnadelberg in #241.