Update contributing guide
Update contributing guide

* Replace the mailing list link with Discuss. The mailing list has not been used for a few years.
* 'make format' no longer exists, so replace it with 'gofmt'
* Remove the instructions about running all tests locally, and debugging flakes.
* Update note about vendoring of dependencies.
* Add link to developer docs.

hashicorp/consul,hashicorp/consul,hashicorp/consul,hashicorp/consul