Documentation: install/upgrade/uninstall guides + recipes incl. Zerto

Adds a docs/ folder under the repo root with full operator documentation
aimed at sysadmins (not webhook developers). The Zerto pre/post script
recipe is the canonical "why does this exist" walkthrough; the GitHub
HMAC, AD password reset, and UI-on-desktop recipes round out common
patterns.

Pages:
- README.md (index)
- concepts.md (5-minute "what is a webhook" explainer)
- installation.md (interactive + silent install)
- upgrading.md (single-click upgrade flow + edge cases)
- uninstalling.md (clean removal + wiping ProgramData)
- runas-modes.md (Service / InteractiveUser / SpecificUser decision flow)
- service-account-and-ad.md (gMSA setup, delegated rights)
- network-and-security.md (bind addresses, allowlists, HTTPS, secret storage)
- troubleshooting.md (symptom -> first check, common errors)
- recipes/zerto-pre-post-scripts.md (canonical use case)
- recipes/github-style-hmac.md (GitHub / Stripe-shaped webhooks)
- recipes/ad-password-reset.md (gMSA-backed self-service reset)
- recipes/ui-on-desktop.md (InteractiveUser pattern)

Top-level README.md restructured to point at docs/ as the source of
truth, dropping the duplicated installation snippets.

Installer ships docs/ alongside the binaries so they're available
offline at C:\Program Files\WebhookServer\docs\. GUI Help menu gains
a "Documentation" item that opens the docs site in a browser.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/WebhookServer.Gui/MainWindow.xaml

@@ -57,6 +57,8 @@
                 <MenuItem Header="_Restart service" Command="{Binding RestartServiceCommand}"/>
             </MenuItem>
             <MenuItem Header="_Help">
+                <MenuItem Header="_Documentation…" Command="{Binding OpenDocumentationCommand}"/>
+                <Separator/>
                 <MenuItem Header="_About Webhook Server…" Command="{Binding ShowAboutCommand}"/>
             </MenuItem>
         </Menu>

src/WebhookServer.Gui/ViewModels/MainViewModel.cs

@@ -290,6 +290,23 @@ public sealed partial class MainViewModel : ObservableObject
         dlg.ShowDialog();
     }
 
+    [RelayCommand]
+    private void OpenDocumentation()
+    {
+        try
+        {
+            System.Diagnostics.Process.Start(new System.Diagnostics.ProcessStartInfo
+            {
+                FileName = "https://github.com/recklessop/webhook-server/tree/main/docs",
+                UseShellExecute = true,
+            });
+        }
+        catch (Exception ex)
+        {
+            ShowError("Could not open documentation", ex);
+        }
+    }
+
     [RelayCommand]
     private void Exit()
     {