Add Timeline documentation pages (about, guide, tech)

3 new public pages matching the korrespond doc pattern: timeline-about.php
(marketing), timeline-guide.php (user guide), timeline-tech.php (tech
showcase). Hero images + 4 screenshots in assets/images/timeline/. Doc
links added to timeline.php and preview.php?tool=timeline.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

timeline.php

@@ -15,6 +15,14 @@ require_once __DIR__ . '/includes/layout.php';
                         <button type="button" class="lang-btn" data-lang="pl">&#127477;&#127473; PL</button>
                     </div>
 
+                    <div class="korr-doc-links">
+                        <a href="/timeline-about.php" target="_blank">About this tool</a>
+                        <span aria-hidden="true">&middot;</span>
+                        <a href="/timeline-guide.php" target="_blank">User guide</a>
+                        <span aria-hidden="true">&middot;</span>
+                        <a href="/timeline-tech.php" target="_blank">How it works</a>
+                    </div>
+
                     <div class="control-row" id="timelineEngineControl">
                         <span class="control-label" data-i18n="timelineEngine">Engine</span>
                         <label><input type="radio" name="timelineEngine" value="azure_mini" checked id="timelineEngineAzureMini"> <span data-i18n="timelineEngineAzureMini">Azure gpt-4o-mini</span> &#9733; <small class="control-hint">(fast)</small></label>