Rewrite HTTP server definitions section

[lmolkova]

Fixes #414

Changes

• rewrites HTTP server definitions section: removes unused terminology and focuses on HTTP framework instrumentation experience
• adds example diagrams
• Removes repetitive notes from yaml replacing them with links

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• CHANGELOG.md updated for non-trivial changes.
• schema-next.yaml

[trask]

I think this is very helpful

[lmolkova]

After discussion with @trask:

• removed request-target in absolute form from options. Based on the RFC7230 "A client MUST send a Host header field in an HTTP/1.1 request even if the request-target is in the absolute-form", so there is no need in mentioning it
• removed framework/web-server options. The motivation:
  • the server does not know the original domain name (e.g. when the request is made against foo.io and is handled by wus3-foo.cloudapp.azure.com, the latter one does not need to know the original DNS name the client reached).
  • if there is some configuration, this is an HTTP proxy/lb configuration and would likely be different than the original DNS name

Also, added :authority HTTP/2 pseudo-header which is interchangeable with HTTP1.1 Host and may be set without the Host header. If both are set, they SHOULD match.