[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.

Ouvrez docs/user-guide/nodes.md et repérez la section consacrée à l’Inject node. Examinez la formulation actuelle concernant le scheduler et ajoutez une clarification ainsi qu’un lien pertinent vers des no-core nodes qui gèrent la planification lors des pannes de courant et des redémarrages ; le guide ne doit plus laisser entendre qu’un core scheduler node existe.