[user-guide] Inject node description improvement and related links

Context

• Guide URL: https://nodered.org/docs/user-guide/nodes
• Source URL: https://github.com/node-red/node-red.github.io/blob/master/docs/user-guide/nodes.md
• Section: Inject node
• Text snippet:

If you are looking at intervals greater than one day - consider using a scheduler node that can cope with power outages and restarts.

Problem or Improvement

In this section, it is said: ...consider using a scheduler node....

This is mostly unclear for the newcomer who reads the guide, as:

1. There is no scheduler core node type.
2. There is no reference or link to no-core nodes which fulfill such task, so the user can further research that option if needed.

Suggestion

Update the node description to tackle on this unclear reference.

Open docs/user-guide/nodes.md and locate the Inject node section. Review the current scheduler wording and add clarification plus a relevant link to no-core nodes that handle scheduling across power outages and restarts; the guide should no longer imply that a core scheduler node exists.