feat: add documentation for sending custom notification events and se…

[wakonig]

related to bec-project/bec#928

[github-actions]

Remaining TODOs in the docs:

docs/how-to/devices/add-an-epics-motor.md:<!-- TODO: Add video here -->
docs/how-to/general/send-messages-to-signal.md:<!-- TODO: Uncomment once we have a better support for stickers -->
docs/getting-started/quick-start/01-open-bec.md:<!-- TODO: link to different application modes. -->
docs/getting-started/quick-start/01-open-bec.md:<!-- TODO: link to information about deployments. -->
docs/getting-started/quick-start/04-run-your-first-scan.md:<!-- TODO: link to learn about signals -->
docs/getting-started/next-steps/script-gui-interactions.md:TODO: Revisit as a dedicated, safety-reviewed scripting tutorial.