Document ssh shell integration

[jparise]

Now that all of our shell integrations provide ssh features (ssh-env and ssh-terminfo), we should expand our features/shell-integration documentation to describe how they work.

We should also specifically note that we forward the COLORTERM, TERM_PROGRAM, and TERM_PROGRAM_VERSION environment variables when the ssh-env feature is enabled, but that requires the remote sshd configuration to allow those variables to be applied to the remote session.

[jparise]

This is also covered the the ssh-env-specific configuration documentation:

• https://github.com/ghostty-org/ghostty/blob/7c7d5421f9380f7ca65a0b413d3117d5a88e2dd1/src/config/Config.zig#L2719-L2724
• https://ghostty.org/docs/config/reference#shell-integration-features