[codex] Add timer and scheduled workflows guide

[sfmskywalker]

Summary

• add a dedicated timer and scheduled workflows guide for Delay, StartAt, Timer, and Cron
• correct the trigger guide wording and link the new page from the running workflows navigation
• update the documentation backlog inventory for the completed DOC-040 slice

Validation

• verified activity and scheduler behavior against release/3.8.0 in elsa-core and elsa-extensions
• ran git diff --check
• ran a relative markdown link existence check for the changed docs