Add opentelemetry.io docs

[austinlparker]

Per open-telemetry/opentelemetry.io#472, we're mirroring the docs content on the website to each SIG. When a release occurs and these docs are updated, please make an issue or PR mirroring them to their appropriate location in the website repo (https://github.com/open-telemetry/opentelemetry.io/tree/main/content/en/docs/php).

[codecov]

Codecov Report

Merging #291 (59f555b) into main (379b6ff) will not change coverage.
The diff coverage is n/a.

❗ Current head 59f555b differs from pull request most recent head 08376f1. Consider uploading reports for the commit 08376f1 to get more accurate results

@@            Coverage Diff            @@
##               main     #291   +/-   ##
=========================================
  Coverage     94.16%   94.16%           
  Complexity      493      493           
=========================================
  Files            47       47           
  Lines          1183     1183           
=========================================
  Hits           1114     1114           
  Misses           69       69           

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