readme badges and cleanup (#1053)

* adding badges to the various readme files to make more obvious the current versions
* list all packages and versions in main readme
* remove old README documentation (excluding development-specific), and direct users to opentelemetry.io
* link to opentelemetry.io where possible in package readme
* adding a badge to link to releases
releases are not easy to find, since they exist only in the subtree split repository

Author: brettmc

README.md

@@ -1,7 +1,17 @@
+[![Releases](https://img.shields.io/badge/releases-purple)](https://github.com/opentelemetry-php/sdk/releases)
+[![Source](https://img.shields.io/badge/source-sdk-green)](https://github.com/open-telemetry/opentelemetry-php/tree/main/src/SDK)
+[![Mirror](https://img.shields.io/badge/mirror-opentelemetry--php:sdk-blue)](https://github.com/opentelemetry-php/sdk)
+[![Latest Version](http://poser.pugx.org/open-telemetry/sdk/v/unstable)](https://packagist.org/packages/open-telemetry/sdk/)
+[![Stable](http://poser.pugx.org/open-telemetry/sdk/v/stable)](https://packagist.org/packages/open-telemetry/sdk/)
+
 # OpenTelemetry SDK
 
 The OpenTelemetry PHP SDK implements the API, and should be used in conjunction with contributed exporter(s) to generate and export telemetry.
 
+## Documentation
+
+https://opentelemetry.io/docs/instrumentation/php/sdk/
+
 ## Getting started
 
 ### Manual setup