Update jjui theme docs links in README.md

[lehnerpat]

Hey, thanks for creating this great resource of themes for jjui! ❤️

The URL of the themes documentation page for jjui has changed. This PR updates the README to use the current link.

It would be great if you could also update the link that's shown on the top right of the repo main page, under the description 🙏

[Copilot]

Pull request overview

Updates the README’s jjui theme documentation links to the new upstream documentation URL, keeping this theme repository’s entry points current for users.

Changes:

• Replaced the outdated https://idursun.github.io/jjui/Themes.html link with https://idursun.github.io/jjui/customization/themes/ in the README title line.
• Updated the “overrides” link in the Customization section to the same new documentation page.

Note: the request to update the “top right” link on the repository main page (GitHub repo homepage/description link) can’t be done via this PR’s code changes; it needs a repository settings update by a maintainer.

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