Update uninstall instructions

[youngnick]

Now that we are supporting multiple installation methods (YAML, Helm, and alpha Operator), we need to update the uninstall instructions.

Currently, they also do not handle cluster-scoped resources like RBAC and CRDs, so we need to have instructions for what to do there in the removal case.

[skriss]

The uninstall instructions are here:

• (rendered) https://projectcontour.io/docs/main/deploy-options/#uninstall-contour
• (source) https://github.com/projectcontour/contour/blame/main/site/content/docs/main/deploy-options.md#L351

It would be great to have uninstall steps that fully remove the installation, including any cluster-scoped resources, for each method documented in:

• https://projectcontour.io/getting-started/#install-contour-and-envoy