Update Vale style guide to match Grafana's

[heitortsergent]

The Writer's Toolkit has a Vale implementation that includes the Google style guide, along with some custom rules under the Grafana folder.

We should:

• Update the Vale folder in the k6-docs repo to match the Writer's Toolkit folder
• Test the workflow to make sure that PRs can still pass
  • Having more rules means that more PRs may fail. We might need to edit the Vale lint action to not fail for some time while we fix any existing vale warnings.