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.

Besides, since translations in other languages are expected to be placed
under 'src', all original guide content has been moved into 'src/en'.

[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"]