Proposal: Add Style section on using parts & exportparts attribute

Nobody has claimed this yet.

When styling a LitElement, I wanted to expose some elements using parts, and it was a bit challenging to find the right information or examples.

Proposal is to add a section on using the `part` and `exportparts` attribute in the styles page.

Impact is users of Lit looking to expose an element in the shadow DOM for arbitrary styles can find a technique on our Styles page and be pointed in the right direction. Another impact is that my naive googling in the past to solve this problem always results in a suggestion of using `createRenderRoot`, and finding `part` on the styling page would have been preferred.

Start from the Styles page in the Lit website documentation and review how it presents styling techniques. Add a section covering the part and exportparts attributes, with guidance or examples for exposing shadow-DOM elements to arbitrary styles; it is done when users can find this technique there.

Written by the indexing model from the issue text.