docs: add developer design/architecture guide (EN/JA) (#251)

* docs: add developer design/architecture guide (EN/JA)

The docs site covered features and reference but had no entry point for
the codebase's design. Add a Developer Guide section describing the
module structure (engine shared kernel, platform-paper, platform-velocity),
the protocol-version compatibility model, and the Service Container /
Feature Gating pattern, so contributors can understand the architecture
without reading the source first.

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

* docs: mirror cross-server DM docs into English (#231)

PR #231 added the cross-server direct messaging feature but updated only
the Japanese docs. Port the same additions to the English pages
(configuration, direct-message, velocity, commands) so both locales stay
in sync.

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

* style: apply biome formatting to ja.ts sidebar config

The developer-guide sidebar entries were not biome-formatted, failing the
build_docs CI check (format:check). Apply the formatter.

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

---------

Co-authored-by: Claude <noreply@anthropic.com>

2 files changed, 62 insertions, 0 deletions

diff --git a/website/.vitepress/config/en.ts b/website/.vitepress/config/en.ts
index f9929fd..26076c9 100644
--- a/website/.vitepress/config/en.ts
+++ b/website/.vitepress/config/en.ts
@@ -73,6 +73,37 @@ export const en: DefaultTheme.Config = {
           },
         ],
       },
+      {
+        text: 'Developer Guide',
+        items: [
+          {
+            link: '/docs/developers/introduction',
+            text: 'Introduction',
+          },
+          {
+            text: 'Design / Architecture',
+            link: '/docs/developers/architecture',
+            items: [
+              {
+                link: '/docs/developers/engine',
+                text: 'engine - Shared Kernel',
+              },
+              {
+                link: '/docs/developers/platform-paper',
+                text: 'platform-paper - Paper / Folia Plugin',
+              },
+              {
+                link: '/docs/developers/platform-velocity',
+                text: 'platform-velocity - Velocity Plugin (Proxy Relay)',
+              },
+            ],
+          },
+          {
+            link: '/docs/developers/resource',
+            text: 'Build, Release & Versioning',
+          },
+        ],
+      },
     ],
   },
 };
diff --git a/website/.vitepress/config/ja.ts b/website/.vitepress/config/ja.ts
index 12fd9a1..1f8f4c0 100644
--- a/website/.vitepress/config/ja.ts
+++ b/website/.vitepress/config/ja.ts
@@ -73,6 +73,37 @@ export const ja: DefaultTheme.Config = {
           },
         ],
       },
+      {
+        text: '開発者向けガイド',
+        items: [
+          {
+            link: '/ja/docs/developers/introduction',
+            text: 'はじめに',
+          },
+          {
+            text: '設計/アーキテクチャ',
+            link: '/ja/docs/developers/architecture',
+            items: [
+              {
+                link: '/ja/docs/developers/engine',
+                text: 'engine - 共通カーネル',
+              },
+              {
+                link: '/ja/docs/developers/platform-paper',
+                text: 'platform-paper - Paper / Folia プラグイン本体',
+              },
+              {
+                link: '/ja/docs/developers/platform-velocity',
+                text: 'platform-velocity - Velocity プラグイン本体 (プロキシ中継)',
+              },
+            ],
+          },
+          {
+            link: '/ja/docs/developers/resource',
+            text: 'ビルド・リリース・バージョニング',
+          },
+        ],
+      },
     ],
   },
 };