doc: 增加 Affix 文档

Author: ArgoZhang

src/BootstrapBlazor.Server/Components/Samples/Affixs.razor

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+@page "/affix"
+
+<h3>@Localizer["AffixTitle"]</h3>
+
+<h4>@Localizer["AffixIntro"]</h4>
+
+<DemoBlock Title="@Localizer["AffixNormalTitle"]" Introduction="@Localizer["AffixNormalIntro"]" Name="Normal">
+    <div style="overflow: auto; height: 200px;">
+        <div style="height: 50px;">标题栏 Header</div>
+        <Affix>
+            <div style="line-height: 50px;">固定行 Affix</div>
+        </Affix>
+        <div style="height: 300px;">
+            <p>向下滚动查看效果</p>
+            <div>Scroll down to see the affix effect</div>
+        </div>
+    </div>
+</DemoBlock>
+
+<DemoBlock Title="@Localizer["AffixPositionTitle"]" Introduction="@Localizer["AffixPositionIntro"]" Name="Position">
+    <div style="overflow: auto; height: 200px;">
+        <div style="height: 50px;">标题栏 Header</div>
+        <div style="height: 300px;">
+            <p>上下滚动查看效果</p>
+            <div>Scroll up and down to see the effect</div>
+        </div>
+        <Affix Position="AffixPosition.Bottom" Offset="10">
+            <div>底部固定行 Affix</div>
+        </Affix>
+    </div>
+</DemoBlock>

src/BootstrapBlazor.Server/Components/Samples/Affixs.razor.cs

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+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the Apache 2.0 License
+// See the LICENSE file in the project root for more information.
+// Maintainer: Argo Zhang([email protected]) Website: https://www.blazor.zone
+
+namespace BootstrapBlazor.Server.Components.Samples;
+
+/// <summary>
+/// Affixs 组件
+/// </summary>
+public partial class Affixs
+{
+    [Inject]
+    [NotNull]
+    private IStringLocalizer<Affixs>? Localizer { get; set; }
+}