Header improvements

[hudochenkov]

• On Demo page header is much narrow and links are different (they have hover).
• Make “Demo” in the header looks like other links.
• There is no hover state for header links.

I agree to fix the strange style. But this is due to the siteNavGroupActive class name by Docusaurus internal logic, it doesn't seem that we can disable this class name… 😕

See core/nav/HeaderNav.js in Docusaurus code:

• https://github.com/facebook/docusaurus/blob/e5e085b33d1d9b36a6470847579fd7fee68a12b9/packages/docusaurus-1.x/lib/core/nav/HeaderNav.js#L199
• https://github.com/facebook/docusaurus/blob/e5e085b33d1d9b36a6470847579fd7fee68a12b9/packages/docusaurus-1.x/lib/core/nav/HeaderNav.js#L220-L221

Originally posted by @ybiquitous in #67 (comment)