[DevHub] Add `Update URI` theme header info to Main Stylesheet doc

[ironprogrammer]

Issue Description

The Main Stylesheet (style.css) page does not include information about the optional Update URI theme header.

URL of the Page with the Issue

• Main Stylesheet (style.css)

Section of Page with the issue

• Example
• Explanations

Why is this a problem?

Introduced with WordPress 6.1, this optional header allows theme authors to indicate a custom URI for theme updates, or to disable update checks altogether by setting it to false. It would be helpful to include information about this header in the docs.

Suggested Fix

• Include an example usage of Update URI in the code sample section, such as Update URI: https://example.com/my-theme/, or Update URI: false.
• Include a description of the "Update URI" header in the explanations section, including a link to the dev note where this was introduced.
• Optionally position the false option as a new section on "How to disable theme updates".

[github-actions]

Heads up @WordPress/docs-issues-coordinators, we have a new issue open. Time to use 'em labels.