Remove `<header>` in official site

[Cyrus Yip]

Is your feature request related to a problem? Please describe.
I have to scroll over <header> to read the docs. When I switch tabs, I need to scroll again. It's inconvenient.

Describe the solution you'd like
Remove the whole <header>. The website will look like this:

---

This content can be added under navigation bar.

[vaughan]

I'm not a web developer, this is kind of out of my scope. Should be a simple PR if anyone wants to try it