Add template workflow to check Markdown files for problems

[per1234]

On every push and pull request that affects relevant files, and periodically, check the repository's Markdown files for
problems:

• Use markdownlint-cli to check for common problems and formatting.
• Use markdown-link-check to check for broken links.

The Arduino tooling Markdown style is defined by the .markdownlint.yml file.

In the event the repository contains externally maintained Markdown files, markdownlint can be configured to ignore them via a .markdownlintignore file:
https://github.com/igorshubovych/markdownlint-cli#ignoring-files

markdown-link-check is configured via the .markdown-link-check.json file:
https://github.com/tcort/markdown-link-check#config-file-format

The workflow is also used for this repository's own CI.

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Notes

Failure of the link check CI is expected currently due to:

• Links based on the final repository location under the arduino organization.
• Link to a readme for a template workflow that has not yet been added to the repository.

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I have always been bothered by the lack of compatibility of our link check task with Windows.

The code used to iterate over the repository's Markdown files is in Bash. Normally this works perfectly well on Windows due to Task using a built-in shell interpreter independent from whatever is in use natively on the machine running the task. However, this particular Bash code is run by npx --call, which uses the native shell of the operating system. This means the Bash code is run in cmd on Windows machines, which doesn't work. I have spent a ton of time trying to find a way to get this working with npx --call on Windows:

• Attempting to write an equivalent in batch, which would be easy enough in a batch file, but is not at all once you are faced with the odd restrictions imposed by passing the code as an argument to npx in combination with some mangling done by YAML and Task.
• Looking for a way to configure npx to use another shell, which seems to be impossible.

The solution I settled on is to simply abandon the use of npx on Windows, instead requiring Windows users to have markdown-link-check pre-installed and in PATH. This allows the use of the Bash code directly in the task. Other operating systems continue to use the traditional npx approach. Even though this isn't an ideal solution, it's a great improvement over the previous one where it was impossible for Windows contributors to run the task.