make the Governance page readable straight through

[SethTisue]

this is a sequel to #1700

@tgodzik told me in conversation that he is open to a plain-text look instead of the cards

I think the cards made sense when they were at the top of https://www.scala-lang.org/community/ and it was important not to use too much vertical real estate

but now I think it's merely annoying to have click through all the cards in order to read everything

also, having everything in cards meant the page had no proper table of contents

in putting this PR together, I tried to avoid changing the text, but I did do some light editing/rearranging to reduce redundancy between the "Who's behind Scala?" sections and the other sections, to make the page make better sense as a single whole

[SethTisue]

the old look is currently still viewable at https://www.scala-lang.org/governance/

and here are screen shots showing the plain-text look I'm suggesting. (UPDATE: the texts have changed a bit since these screen shots, so please use the diffs to review the actual texts)