User-defined model aliases (config.yaml model_aliases: and
model.aliases.*) have worked since early versions but were entirely
undocumented. Add a dedicated 'Custom model aliases' section to
slash-commands.md covering both YAML config formats and the
'hermes config set' shell form, mirror a shorter version into the
configuring-models 'Alternative methods' section, and cross-link from
the two /model table rows.
Flagged by @weehowe on Twitter — he wasn't aware the feature existed.