Document and support using HTTP/2 with NGINX

What does this Merge Request do?

This MR does the following:

• Breaks out the NGINX steps from the https.md into it's own nginx.md
• Update https.md to include how to configure NGINX to use HTTP/2
• Adds a new http2.md document that points to enabling HTTP/2 in https.md
• Adds a new http.md document that points to installing and configuring NGINX in nginx.md
• Adds support into gdk.yml for easily enabling HTTP2 support for NGINX
• Adds missing mkcert -install step
• Tidies up the formatting of nginx.conf.erb

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• [-] gdk doctor test added, if needed.

Closes #872 (closed) #699 (closed)