docs: consolidate (#204)

* docs: migrate custom http client example

* docs: link README to new client doc, lint and check vs docs style guide

* docs: merge information from docs into README.md

* docs: change note, update indentation

* docs: spelling

* docs: add Go 1.20

* docs: api keys example indentation

* docs: incorporate PR feedback

* docs: reference the Prisma sample, not an old csharp repo

* chore: heading level

* chore: remove old docs reference

Author: stern-shawn

CONTRIBUTING.md

@@ -21,7 +21,7 @@ it can be.
 ## <a name="question"></a> Got an API/Product Question or Problem?
 
 If you have questions about how to use `twilio-go`, please see our
-[docs][docs-link], and if you don't find the answer there, please contact
+[docs](./README.md), and if you don't find the answer there, please contact
 [[email protected]](mailto:[email protected]) with any issues you have.
 
 ## <a name="issue"></a> Found an Issue?
@@ -55,10 +55,6 @@ you're working on.
 For large fixes, please build and test the documentation before submitting the
 PR to be sure you haven't accidentally introduced layout or formatting issues.
 
-If you want to help improve the docs at
-[https://www.twilio.com/docs/libraries/go][docs-link], please contact
-[[email protected]](mailto:[email protected]).
-
 ## <a name="submit"></a> Submission Guidelines
 
 ### Submitting an Issue
@@ -143,6 +139,5 @@ you are working:
 * All classes and methods **must be documented**.
 
 
-[docs-link]: https://www.twilio.com/docs/libraries/go
 [issue-link]: https://github.com/twilio/twilio-go/issues/new
 [github]: https://github.com/twilio/twilio-go