Document downloadJson modebar button

[SatvikMishra08]

Summary

• Documents the opt-in downloadJson modebar button shipped in plotly.js (#7990 / plotly.js).
• Adds downloadJson to the modebar button list and a minimal modeBarButtonsToAdd: ['downloadJson'] config example.

Closes #469

Test plan

• Confirm Configuration Options page shows the new "Download Plot as JSON" example
• Confirm Remove ModeBar Buttons list includes downloadJson
• CI front-matter / order checks pass

[camdecoster]

Thanks for the PR! This change won't actually work until we update the underlying version of plotly.js. That's on my TODO list (in #473). This will be covered there, so I'm going to close this PR. Please let us know if you find any other undocumented gaps.