Document how to use additional local Vale rules

What does this MR do?

For some time, I've wanted to ability to use extra Vale rules above and beyond the officially-approved ones we have in doc/.vale/gitlab, without accidentally committing them back upstream. I'm not sure whether the right approach here would be to append to the global .gitignore in the root, or to add a new one in the proposed doc/.vale/local folder. On original submission, this MR contains both.

From everyone else's perspective, doc/.vale/local is empty.

I'd originally hoped to quietly build this for myself and not share it out, but I realized specifying the new local style required a change to .vale.ini.

Sending this over to @eread and @marcel.amirault for thoughts. Tagging @rdickenson as well, because I have a vague memory that he wanted to use a couple of extra rules, too:

• can you think of any performance issues this might cause?
• which .gitignore approach is the right one?
• should I be barking up a wholly different tree instead? Does another solution exist somewhere, given that I've failed to make extra, personal rules Just Work in VS Code?

Thoughts?

To be extra special clear: this idea MR shouldn't merge without talking to more people on the team. I don't want to give the appearance of sneaking this change in.

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