Lint Documentation MD files on CI

[marcelstanley]

📚 Context

The node docs is comprised of several MDfiles, either manually created or automatically generated.

Sometimes a developer might make changes that are not formatted right and such mistakes go unnoticed.

This is a follow-up to #266

✔️ Solution

Consider enabling markdown at .golangci.yml.

If not possible, add a linter action to CI.

Linters below may be considered:

• https://github.com/DavidAnson/markdownlint
• https://github.com/tcort/markdown-link-check

📈 Subtasks

• Add MD linter action to CI
• Add corresponding recipe to Makefile
• Disable linter rules on non-documentation files