[Docs] missing instructions to set up vale-ignore for md

I discovered while trying to use the `vale-ignore` role (which is mentioned in the docs) to ignore a single use of the word "manpage", which I don't want to add to the custom word list, that it's not set up by default for markdown and the instructions only mention setting it up for rst. It would be nice to have the corresponding conf.py snippet in markdown as well as rst.