Since we moved the docs to integrations, we don't need to build the
docs.zip file anymore, and to make it easier for plugin developers we
move the logic for building the new .docs directory to the `generate'
Makefile target.

Since the workflow relies on the build-docs target existing in the
Makefile to ensure we don't commit a change without calling it, and
since we removed it in the previous commit to fold it in generate, we
don't need it anymore so we can remove it.

Since the `make generate' call is already required if modifying the
docs, and we check that during validate, this is safe to remove.

Since we have merged `build-docs' with `generate', we don't need to
manually invoke `packer-sdc renderdocs' before the old build-docs.

Since the ci-make-docs target is no longer part of the Makefile, we
cannot rely on it for making the latest docs, and therefore we must
remove that from the goreleaser file too.

The docs.zip file should not be part of the release as it is no longer
generated nor required to build the docs for a plugin.

Keeping this in the releaser config caused the release to fail before
the artifacts were updated, leading to a corrupted release.