Convert gitbook format to vuepress

[vordimous]

• update a zilla.yaml reference page with more details and quick links
• configure the necessary nav links and frontmatter
• Polish the build-todo-app page; convert img, hint, code blocks
• Add vuepress structure and hope theme

[vordimous]

@aklivity/engineering, you can ignore everything under the .vuepress directory unless you are familiar/curious about the SSG.