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

README.md

@@ -1,3 +1,9 @@
+[![Releases](https://img.shields.io/badge/releases-purple)](https://github.com/opentelemetry-php/context/releases)
+[![Source](https://img.shields.io/badge/source-context-green)](https://github.com/open-telemetry/opentelemetry-php/tree/main/src/Context)
+[![Mirror](https://img.shields.io/badge/mirror-opentelemetry--php:context-blue)](https://github.com/opentelemetry-php/context)
+[![Latest Version](http://poser.pugx.org/open-telemetry/context/v/unstable)](https://packagist.org/packages/open-telemetry/context/)
+[![Stable](http://poser.pugx.org/open-telemetry/context/v/stable)](https://packagist.org/packages/open-telemetry/context/)
+
 # OpenTelemetry Context
 
 Immutable execution scoped propagation mechanism, for further details see [opentelemetry-specification][1].