[Documentation] How does the `html` tag efficiently render (concepts)

Nobody has claimed this yet.

### Context

I found it very helpful to read over the **concepts** section of the legacy lit-html website: https://lit-html.polymer-project.org/guide/concepts

This goes through the concepts of how the `html` tag uses string literals and expressions with the `` tag to render efficiently, and then update efficiently.

Do we have a similar section in lit.dev?

### Completion criteria (one of):

- Add a lit-html `html` tag concept section on lit.dev?
- It maybe already exists or we don't need it. Close issue with link to page or other issue.

Start by reading the legacy concepts section at https://lit-html.polymer-project.org/guide/concepts and searching lit.dev for an equivalent explanation of the html tag. Done means either adding a comparable lit-html concepts section on lit.dev or confirming that an existing page or another issue covers it, then linking to it and closing the issue.

Written by the indexing model from the issue text.