feat: Partial preserve on refresh

[caalador]

Add documentation on the
possibility to preserve
route partially for refresh.

documents vaadin/flow#21139

[github-actions]

AI Language Review

In the modified Preserving the State on Refresh document, a new section titled "Partial View Chain Preservation" has been added. Within this section, a minor improvement is suggested:

• Use consistent formatting for terminology. The term "url" should be capitalized as "URL" to maintain consistency with the rest of the document, where "URL" is capitalized in other instances (e.g., "new URL in the same client window context").