Silverblue/Kinoite/Sericea: Remove usage of "immutable" in titles / main descriptions
The term immutable is confusing for users, especially for non-technical users that do not understand what it means.
We should remove this term from the pages and try to use easier to understand terms.
Example from https://fosstodon.org/@AdrianVovk/110919053866221179:
The GNOME desktop experience you know and love, built on a new technology stack designed to keep your computer working with minimal effort
We should also remove the variant name duplication in the title for the Silverblue and Sericea pages.
Pages impacted: