Update docs guidelines and skills to clarify applies_to usage

### Collaboration model

### Point of contact.

**Main contact:** @elastic/docs-tech-leads

### Description

Amend the documentation guidelines and relevant skills so it's explicit that applies_to tags should not be used on links/xrefs when the target page already provides that context and it's not critically important on the links/xrefs. Ensure the guidelines cover principles discussed ("let the target page do the heavy lifting"), and consider introducing examples and clarifications for common scenarios.

See https://github.com/elastic/docs-content/issues/6637 for additional context/guidance

### Which deployment methods does this change impact?

### Feature differences

### Resources

Reference: The agreement from the Slack conversation, Ty's point about dimensions for Kibana/Stack, and work done on previous PRs. See examples in https://github.com/elastic/docs-content/pull/6634/changes and commits around issues with applies_to tag misuse.

### Serverless release

### What Elastic Stack release is this request related to?

[View original Slack conversation](https://elastic.slack.com/archives/C05PP2LEC1X/p1779381341753069?thread_ts=1779381341.753069&cid=C05PP2LEC1X)