doc: add alt attribute

Author: ArgoZhang

src/BootstrapBlazor.Server/Components/Layout/HomeLayout.razor

@@ -11,7 +11,7 @@
     <div class="bb-foundation">
         <div class="bb-foundation-content">本项目属于 <a href="https://www.dotnetfoundation.org/">.NET 基金会</a>，并根据其 <a href="https://www.dotnetfoundation.org/code-of-conduct">行为准则</a> 运作</div>
         <a class="bb-foundation-img d-none d-sm-block" href="https://www.dotnetfoundation.org/">
-            <img src="@WebsiteOption.Value.GetAssetUrl("images/dotnet_foundation_v4.png")" width="100" />
+            <img src="@WebsiteOption.Value.GetAssetUrl("images/dotnet_foundation_v4.png")" alt="dotnet-foundation" width="100" />
         </a>
     </div>
     <div class="footer-body">

src/BootstrapBlazor.Server/Components/Layout/MainLayout.razor

@@ -21,7 +21,7 @@
     <DialButton DialMode="DialMode.Radial" Icon="fa-solid fa-gear" Radius="100" Placement="Placement.BottomEnd" class="bb-dial-gear">
         <ButtonTemplate>
             <div class="btn-circle btn-fade dial-button-gear">
-                <img src="@WebsiteOption.Value.GetAssetUrl("images/logo.png")" />
+                <img src="@WebsiteOption.Value.GetAssetUrl("images/logo.png")" alt="logo" />
             </div>
         </ButtonTemplate>
         <ChildContent>

src/BootstrapBlazor.Server/Components/Pages/Chats.razor

@@ -37,7 +37,7 @@
                 <div class="msg-role">
                     @if (message.Role == ChatRole.User)
                     {
-                        <img src="@WebsiteOption.Value.GetAssetUrl("favicon.png")">
+                        <img src="@WebsiteOption.Value.GetAssetUrl("favicon.png")" alt="avatar">
                     }
                     else
                     {