Document how to load Lumo style modules manually (if not using Flow/Hilla)

[rolfsmeds]

Our documentation for Lumo is missing instructions on the need and how to load various Lumo modules manually, if using Vaadin components standalone (Flow would otherwise take care of this by default).

Although our standalone userbase is tiny compared to Flow, we could still add a mention of this to Lumo documentation.

Based on a discussion on Discord: https://discord.com/channels/732335336448852018/1018970318456500295