Use standard markdown header syntax

[Jamie-SA]

I'd like to suggest switching our markdown style to use standard markdown syntax for headers. By this I mean the use of "#" on the title line versus the use of "====" and "----" on the line under it. AFAIK this is the original syntax and I've seen it as the recommended syntax (on some markdown site that I don't remember off the top of my head).

• It is the recommended syntax
• It uses less lines
• It is more uniform when going beyond 2 levels of headers

[rjyounes]

I don't think the pros are particularly compelling, but I don't really care either way; I had chosen that style pretty much at random. We should change the existing markdown files and update .markdownlint.json accordingly.