add TOC to Joining a Project and other .md docs

[weedySeaDragon]

Most people won't scroll down. :-( Sad, but true. They'll typically only read the first paragraph of what they see. And if someone is viewing a doc on a small screen, like a phone or tablet, they see even less.

Adding a Table of Contents (TOC) at the top will get more people to read sections that are later in the document -- that they would have only seen if they scrolled down.

I think this would be really valuable for novices, since they are typically looking for 'a way in,' and don't always know what it is they're looking for. A TOC can provide them with clues. ("Oh! That looks helpful. I'll click and read that section.")

Just add a few bullets for the main sections; nothing long or fancy.

Would be happy to do a PR for each .md document if that'd be helpful.

[tansaku]

@weedySeaDragon shall we make a decision about this PR first? #24 that adds a TOC to the joining a project page ...

[weedySeaDragon]

[Not sure which "this" is the 'this' you're referring to. But I don't think the order is important. They're all in 'continuous improvement' mode.]
#24 looks great

I have the time to go thru the other .md docs if you'd like. (And of course I can't help but think about writing a little task to do it automatically. But that's for another day.)