[DOCS] SDK

[tcarrio]

Using something like ReadTheDocs or similar, provide documentation for usage of the PHP SDK.

[beeme1mr]

Please update the readme to include a reference to these docs once they're available.

https://github.com/open-feature/.github/blob/main/templates/READMEs/README.md#api-reference

[beeme1mr]

We may be able to use something like PHP Documentor and publish the generated docs using GH Pages.