Add documentation on configuring phases with multiple providers

[luyizhao]

Post #638, Nixpacks has support for multiple providers, but it would be helpful to add in the docs how to add custom commands/overrides for phases with multiple providers.

From looking at #638 and my testing, it seems that phases for the first specified provider can be referenced as normal, and second provider custom phase commands can be set by just prefixing the provider name to the phase step, e.g. phases."node:install" (quotes needed for valid toml).

For example:

providers = ["python", "node"]

[phases.install]  # Relative to the first specified provider, python
cmds = ["echo 'first custom install step'"]

[phases."node:install"] # Overrides node provider install
cmds = ["echo 'second custom install step'"]

Thanks!