docs:add documentation for Linode cloud auto-join (#6719)
docs:add documentation for Linode cloud auto-join (#6719)

The go-discover library supports Linode. This adds support for
discovering other Consul agents running on Linode. Consul has supported
this since [66b8c20][1] was merged, so this commit just updates the
documentation to match current features.

[1]: https://github.com/hashicorp/consul/commit/66b8c209904d0b377f3c561c5b21b393bdeb1235
hashicorp/consul,hashicorp/consul,hashicorp/consul,hashicorp/consul