Add a Vale rule to check for direct tab links

What does this MR do and why?

• Add a Vale rule to check for direct tab links
• Update the Docs style guide to explain why this is a warning

Screenshots or screen recordings

Warning in VS Code	Warning in CLI output

(These warnings are not really present in this file, it was just a test!)

How to set up and validate locally

1. Add a link with tab= in it to a doc (e.g, ee/ci/pipelines/downstream_pipelines.html?tab=Multi-project+pipeline)
2. Run Vale against just that file: vale --config .vale.ini --minAlertLevel warning doc/api/dora/metrics.md

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.