Add PR docs validation workflow using GitHub Actions

## What would you like to be added

Add lightweight validation for documentation changes on pull requests in kubernetes/website.

A good initial scope could include basic checks such as:
- Markdown formatting validation
- Link or HTML validation (where applicable)

The exact implementation (e.g., local tooling, CI integration, or existing workflows) can be determined based on what best fits the project.

## Why is this needed

The repository already contains scheduled validation workflows, but having validation tied to pull requests would help catch issues earlier and improve contributor experience.

There is also existing configuration such as `.htmltest.yml` and `linkcheck-config.toml`, which could potentially be reused or extended.

## Comments

I am okay to help implement this in a way that aligns with current project practices.

Review the existing scheduled validation workflows in kubernetes/website, then inspect .htmltest.yml and linkcheck-config.toml for checks that can be reused. Confirm how documentation pull requests are currently handled and choose an approach consistent with project practices. Done means pull requests with documentation changes run the agreed Markdown and link or HTML validation checks.

Written by the indexing model from the issue text.