docs: V24-V25 upgrade guide

[tepi]

Related-to #4574

[mshabarov]

@rolfsmeds @yuriy-fix

we added "Changes In Theming System" sub-chapter to this PR with some overview, but feel free to change the title and supplement the content, also please add the content for Web Components/Flow Components. I suggest to create a new branch on top of this branch and put your content there, this way it will be automatically synchronized with this branch and main.

[mshabarov]

Needs to be rebased to main branch.

[CLAassistant]

All committers have signed the CLA.

[github-actions]

AI Language Review

The new file is complete and quite comprehensive. It covers a wide range of aspects related to upgrading from Vaadin 23 and 24 to newer versions. The information presented appears accurate and well-organized, aligning well with the guidelines for technical documentation.

Overall, the file looks solid, with no apparent areas requiring improvement or adjustment at this point. However, a real-world review would typically involve checking current compatibility and accuracy with live systems and environments, which can't be fully assessed in this static context.

[mshabarov]

@tepi @tltv please sign the CLA.

[mshabarov]

Fixes #4687