ci: doc: link to the generated docs once the CI doc build has run successfully

Problem Description

We have a nice hidden feature that the doc being built for a PR is actually available for download/viewing. It would be nice if that feature could be made more visible (e.g. as proposed by @kartben as an automatic comment).

Proposed Solution

Similar to @kartben 's nice little browser script:

// ==UserScript==
// @name         Show link to CI-built docs for a Zephyr PR
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  add link to CI doc site
// @author       You
// @match        https://github.com/zephyrproject-rtos/zephyr/pull/*
// @grant        none
// ==/UserScript==

(function() {
    'use strict';

    const addDocsLink = () => {
        const PRNumberRegex = /(\d+)/g;
        let targetElement = document.querySelector(".gh-header-title");

        if (targetElement) {
            // Get PR number from URL
            let prNumber = window.location.pathname.split("/").pop();
            if (PRNumberRegex.test(prNumber)) {
                let container = document.createElement('div');

                let docLink = document.createElement('a');
                docLink.href = `https://builds.zephyrproject.io/zephyr/pr/${prNumber}/docs/`;
                docLink.target = "_blank";
                docLink.innerText = `here`;

                container.appendChild(document.createTextNode('CI-built documentation: '));
                container.appendChild(docLink); 
                container.appendChild(document.createElement('br'));

                targetElement.parentNode.parentNode.insertBefore(container, targetElement.parentNode);
            }
        }
    };

    addDocsLink();
})();

Alternatives

./.

Additional context

Has come up several times as a question on Discord plus would probably actively promote active review of doc results by both, contributors and reviewers.