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

[s-makin]

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.

[SecondSkoll]

A lot of the Vale based style guide advice could use a re-work - I'll have a look if I have some time this week. If anyone else picks this up:

• Sally's concern is addressed by mentioning MD format for this ignore, which is:

  {vale-ignore}`something to ignore`
  

• vale-ignore / woke-ignore class based application only works for specific checks (anything with scope: raw will run on all content - even if ignored with a class)
• rst_prolog definition may be unnecessary with the implementation of specific token ignores.