Preliminary support multilingual

mdBook does not natively support multilingual documentation. A common
approach is to use a third-party library like mdbook-i18n-helper, which
relies on a Gettext-based workflow to enable multilingual support.

However, I believe such a complex system is unnecessary for our case.
Therefore, this commit follows the approach suggested in the official
mdBook documentation: overriding the 'index.hbs' template to add a
button that redirects users to the corresponding documentation in
another language, thus achieving basic multilingual support.

This commit introduces the following three files under the theme
directory:

- 'index.hbs': Adds a redirect button for switching languages.
- 'lang-toggle.css': Defines the styling for the button and the dropdown
  menu.
- 'lang-toggle.js': Implements the main logic behind the button's
  behavior.

As a result, two separate books need to be built. However, since
mdBook’s 'book.toml' does not support multiple '[book]' sections in a
single configuration, this commit introduces a configuration template
and a CMake script that generates language-specific toml files for each
version, which are then built into their respective output folders.

[1]: https://github.com/google/mdbook-i18n-helpers

Author: Mes0903

guide/.gitignore

@@ -1 +1,3 @@
 book
+build/*
+book.toml

guide/CMakeLists.txt

@@ -0,0 +1,29 @@
+cmake_minimum_required(VERSION 3.24)
+project(learn-vk-docs)
+
+# Function to configure the book file and add a custom target to build it.
+function(MakeConfig LANGUAGE AUTHORS)
+  # Set the variables for substitution.
+  set(AUTHORS "${AUTHORS}")
+  set(LANGUAGE "${LANGUAGE}")
+  set(ROOT_DIR "${CMAKE_CURRENT_SOURCE_DIR}")
+  set(TARGET_DIR "${ROOT_DIR}/src/${LANGUAGE}")
+
+  # Configure the template file with the substitutions.
+  configure_file(
+    ${ROOT_DIR}/book-template.config
+    ${TARGET_DIR}/book.toml
+    @ONLY
+  )
+
+  # Create a custom target that calls mdbook with the generated config.
+  message("mdbook build ${TARGET_DIR}")
+  execute_process(
+    COMMAND mdbook build -d ${ROOT_DIR}/book/${LANGUAGE}
+    WORKING_DIRECTORY ${TARGET_DIR}
+  )
+endfunction()
+
+# Create configurations for both languages.
+MakeConfig("en" "Karn Kaul")
+MakeConfig("zh-TW" "Mes")

guide/book-template.config

@@ -0,0 +1,10 @@
+[book]
+authors = ["@AUTHORS@"]
+language = "@LANGUAGE@"
+src = "."
+title = "Learn Vulkan"
+
+[output.html]
+theme = "../../theme"
+additional-js = ["../../theme/lang-toggle.js"]
+additional-css = ["../../theme/lang-toggle.css"]