Update documentation in Storybook

[maxatdetroit]

• Update intro docs to indicate how the system should be used (inside an existing website to match surrounding typeface styles) (e.g. https://shoelace.style/)
• Update setup docs to be correct (e.g. https://shoelace.style/getting-started/installation)
• Update theming/customization docs to basically say we don't support theming beyond slot styling to match LightDOM
• Add changelog docs and move documentation on stable vs. experimental builds there (e.g. https://shoelace.style/resources/changelog/)