developer docs "copy" should copy a usable command

[bhaller]

Next in my journey to do a tskit PR. I'm now at https://tskit.dev/tskit/docs/stable/development.html#git-workflow and am trying to set up my repo. It says:

Make sure that your local repository has been configured with an [upstream remote](https://help.github.com/articles/configuring-a-remote-for-a-fork/):

$ git remote add upstream https://github.com/tskit-dev/tskit.git

That last line is inside a box that has a "copy" button. If I use that button, I get this on my clipboard:

$ git remote add upstream https://github.com/tskit-dev/tskit.git

Pasting that into my terminal gives an error:

bhaller@glass % $ git remote add upstream https://github.com/tskit-dev/tskit.git
zsh: command not found: $

Obviously I just need to leave out the $. But what gets copied shouldn't include the $ character in the first place; it isn't particularly useful to put something onto the user's clipboard that is not, in fact, usable. :-> Presumably this issue applies across lots of other places in the doc too.

[petrelharp]

Well this is slightly annoying: in a few places we'd like to put the $ at the start of the line to specify "this is a bash command" (and thus different from output); it'd be nice if jupyterbook provided a way to insert that in a way that doesn't get copy-pasted, but I can't find it. I'll remove all or most of the $s unless someone thinks otherwise.

[bhaller]

Can you put the $ to the left of the box somehow? $ [box...<copy button>], if that makes sense?

[hyanwong]

How about something like this in the config file? (although I don't know how to make it only apply to bash code)

sphinx:
  config:
    copybutton_prompt_text: "$ "
    copybutton_prompt_is_regexp: false
    copybutton_only_copy_prompt_lines: false

Alternatively you might be able to add the $ using a CSS ::before pseudoelement? That's what I've done in the past.

[petrelharp]

That looks promising!