Autodetect and warn the use of HTML in metric markdowns

[ritik-malik]

The rules and the template for creating a metric markdown can be found here.

One of the point says: No HTML code should be used in the metrics, only markdown in preferred.
This is because, in pandoc while converting from a document to PDF, we've to specify the type of document that we are converting (markdown in this case), so if there is HTML used in the document, it could cause unwanted errors.

Catching them in the PDF or the repo itself would require manual work, instead we would like to scan the documents and throw a warning if it finds any HTML embedded.