* docs: Update outdated info about terraform-providers org

* Remove codecov mentions

An update on the deprecated state of vendor provisioners for our contribution guide.

Co-Authored-By: Pam Selle <pamela.selle@gmail.com>

Co-Authored-By: Kristin Laemmert <mildwonkey@users.noreply.github.com>

Alisdair's fixes.
Co-Authored-By: Alisdair McDiarmid <alisdair@users.noreply.github.com>

Co-Authored-By: Alisdair McDiarmid <alisdair@users.noreply.github.com>

Our documentation for how to contribute was in quite a state of disrepair,
with some documents still describing things as they were before moving
providers into separate repositories, others making assumptions about
Go development that are no longer true in modules mode, and so forth.

This is an attempt at a reset to a good state that should work with the
codebase as it currently stands, and should hopefully serve as a basis
for iterative improvement from here.

These new instructions lean primarily on standard Go toolchain usage and
instruct using the Makefile only for some Terraform-specific situations
that the Go toolchain does not automatically handle. The idea here is that
this direct usage of primary commands in the Go toolchain is less likely
to be broken by changes in future Go releases, and should be immediately
familiar to anyone who has experience with Go development.

* website/contributing: update contribution documentation

This PR seeks to remove outdated and incorrect information. There is
still work to be done updating the information that's left; this is
merely the first step.

Adds the Oracle OPC provider to the list of Hashicorp Providers in the
CONTRIBUTING.md document.

This is a vastly expanded and clarified Contributing guide. It includes
guidelines for each type of contribution and issue report that are
designed to be linked off to from issue / PR comments.

We expect the text will continue to evolve as we learn what works and
what doesn't.

This guide will coincide with a HashiCorp blog post explaining the
motivation and context around these changes:

https://github.com/hashicorp/www/pull/160