Work with ContractSpecs in Other Languages (#768)

* initial commit

* setup layout

* contract spec complex and simple examples

* add examples for getting and parsing contract events

* add explanation of a contract spec

* format mdx

* fix url paths

* Editorial and formatting

* add support for rust and PHP in code examples

* fix code samples

* fix merge conflicts

* fix some markdown link syntax

* add PHP language to code highlighting languages

* fine-tune code examples a bit

* resolve PR comments

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Co-authored-by: Bri <[email protected]>
Co-authored-by: Elliot Voris <[email protected]>