Fix empty button role

[choldgraf]

Now when buttons don't have a link specified (e.g. {button}`Text`) it emits a span with .button class instead of a link button that is empty.

• Adds more explicit parsing to button roles so it's easier to understand
• Adds a test for a few different button role conditions
• Fixes Button role with no link shows as an empty button #2061

Right now this won't be styled so this PR adds a style to it:

• Add more types to button style myst-theme#728

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[stefanv]

Instead of two regexs, why not match on the standard pattern: text<link>, and then do the logic of figuring out what to display: if is empty, show text, if text is empty, show link.

[stefanv]

I.e., two groups, one for text, one for link, including triangle brackets. Then can check if link == <> and in that case render nothing for the link.

[choldgraf]

I had split it into two just to be really explicit because the last REGEX I found pretty opaque, but then again I find all regexes opaque, so sounds good I will revert back to a one-liner :-)

[choldgraf]

OK this is now two PRs - one for myst-theme, I updated the body to have links etc. I think the failing test is that flaky DOI test