Fix title tags per page for SEO

[sh1mmer]

We don't have title tags on pages which may be affecting SEO. Ideally each page should have its own title that can be passed from the MDX (or whatever).

I tried adding this to an MDX page <helmet><title>Foo bar</title></helmet> and in this should work via Helmet inheritance but I wasn't able to get it to work.

I suspect it might be an issue with the child relationships between:

• layout/index.js
• layout/Header-<lang>.js
• some MDX page

Since I think both the page and the headers are children of the layout index but I don't know how that Helmet will work with siblings like that.