Adds a copy-to-clipboard button for code blocks in the documentation to improve developer experience.#646

Rishi2600 wants to merge 2 commits intoprobot:masterprobot/probot.github.io:masterfrom

Rishi2600:feat/add-copy-to-clipboard-button-docsRishi2600/probot.github.io:feat/add-copy-to-clipboard-button-docsCopy head branch name to clipboard