Add support for sphinx-copybutton

[rmartin16]

Changes

• Based on Add copy button for code blocks beeware#210, adds support for a copy button for code blocks.
• Ensures all code blocks use the appropriate lexer for the platform's commands.
• Misc cleanup

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

👍

[rmartin16]

@freakboy3742, as a note, since this came up for rubicon. I added the # prompt for Linux root bash sessions; however, this means comments in a code block will get treated as commands. So, we can either remove this proactively now for Briefcase and Toga or just keep it in mind going forward.

[freakboy3742]

@rmartin16 I missed that detail in the noise of regexes... given that we're only using $ for prompts anyway, my inclination would be to remove # from that regex and avoid the potential confusion.