Moving the documentation site to VuePress about typeorm.github.io

[LuoRiWuSheng]

Issue type: documentation issue

I'm proposing to migrate the documentation to use VuePress.

It has the following benefits:

• Fully server-side rendered (better first load time)
• Pre-compiled Vue components (smaller bundle size and better performance)
• Compiles markdown during build (documentation fetched from GitHub pages instead of "Raw")
• Additional features like search

Afterwards, documentation would be able to be directly built and served within this repo. You can host it with either GitHub pages or Netlify (the latter has more feature but needs a brief setup).

Before I file a PR, please answer the following questions:

• Is this migration desirable?
• Is the default theme (used on vuepress.org) desirable, or we should modify that to match our current styles?

good idea!