docs(channels): add Discord adapter documentation (PR #656)
Documents the Discord Incoming Webhook + Interactions adapter:
- Config fields, setup steps (webhook URL + slash-command app),
inbound interaction types (PING/APPLICATION_COMMAND/MESSAGE_COMPONENT),
message splitting at the 2000-char limit, and a DISCORD_WEBHOOK_URL
secret example.
- Updates title/description, Overview, Adapters count, webhook type list,
and Example Configs sections.
Pairs with molecule-core PR #656 / closes #625.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Molecule AI Documentation Specialist
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