[docs] Document the @lit-labs/ssr-dom-shim package on docs/ssr/dom-emulation/

[justinfagnani]

The SSR Dom Emulation page at https://lit.dev/docs/ssr/dom-emulation/ doesn't include the full set of APIs we currently emulate. Nor does it mention what's in the automatic shim vs the @lit-labs/ssr-dom-shim package.