Enable Markdown linting

[ivanvc]

Currently, the repository doesn't have a Markdown linter. This causes PRs sometimes to discuss using Markdown standards and contributes to a drift in discussions. Following what we recently discussed in a PR (https://github.com/etcd-io/website/pull/774#discussion_r1498552258). Enabling linting for the whole repository may require a significant effort. I think a good starting point would be to incrementally run a Markdown linter against the changes from pull requests.