Add Markdown layout and Ubuntu development guide

Introduces a reusable MarkdownLayout.astro component for consistent page layouts and adds a detailed guide (ubuntu.md) for developing OBS Background Removal on Ubuntu, including setup, build, packaging, and linting instructions.

Author: umireon

pages/src/layouts/MarkdownLayout.astro

@@ -0,0 +1,19 @@
+---
+import Layout from './Layout.astro';
+
+interface Props {
+  frontmatter: {
+    lang: string;
+    title: string;
+    description: string;
+  };
+}
+
+const { lang, title, description } = Astro.props.frontmatter;
+---
+
+<Layout lang={lang} title={title} description={description}>
+  <main class="white-box">
+    <slot />
+  </main>
+</Layout>

pages/src/pages/dev/ubuntu.md

@@ -0,0 +1,74 @@
+---
+layout: ../../layouts/MarkdownLayout.astro
+lang: en
+title: How to develop OBS Background Removal on Ubuntu
+description: How to develop OBS Background Removal on Ubuntu
+---
+# How to develop it on Ubuntu
+
+## Install system dependencies
+
+```
+sudo apt install build-essential zsh cmake git curl zip unzip tar
+```
+
+## Clone the source
+```
+git clone https://github.com/royshil/obs-backgroundremoval.git
+cd obs-backgroundremoval
+```
+
+## Install vcpkg
+
+```
+git clone https://github.com/microsoft/vcpkg.git ~/vcpkg
+~/vcpkg/bootstrap-vcpkg.sh
+export VCPKG_ROOT=~/vcpkg
+```
+
+## Install build dependencies with vcpkg
+It takes 10-20 minutes.
+```
+${VCPKG_ROOT}/vcpkg install --triplet x64-linux-obs
+``` 
+
+## Download ONNX Runtime with CMake
+```
+cmake -P cmake/DownloadOnnxruntime.cmake
+```
+
+
+## Build with CI scripts
+
+```
+./.github/scripts/build-ubuntu --target ubuntu-x86_64 --config RelWithDebInfo
+```
+
+## Test plugin with system OBS
+
+```
+sudo cmake --install build_x86_64
+```
+
+## Package the plugin
+
+```
+./.github/scripts/package-ubuntu --target ubuntu-x86_64 --config RelWithDebInfo --package
+```
+
+## Test the package installatino
+
+```
+sudo dpkg -i release/obs-backgroundremoval-*-x86_64-linux-gnu.deb
+```
+
+## Lint
+
+```
+/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"
+eval "$(/home/linuxbrew/.linuxbrew/bin/brew shellenv)"
+brew install obsproject/tools/clang-format@19 obsproject/tools/gersemi
+export PATH="/home/linuxbrew/.linuxbrew/opt/clang-format@19/bin:$PATH"
+./build-aux/run-clang-format
+./build-aux/run-gersemi
+```