Find a better way to use tabpane shortcodes

[chalin]

Part of the problem with the current way in which we're using tabbed panes is that the tools don't know that the content of a tab is code. Consequently we are forced to add checker and linter directives like <!-- markdownlint-disable -->, <!-- prettier-ignore-start -->, etc., for each checker and linter.

I propose that we use code-block fences inside tabs. Here's an illustration of what I mean:

• Use of tabpanes that obviates the need for Prettier/linter-ignore directives #3126

Tasks

• POC: Use of tabpanes that obviates the need for Prettier/linter-ignore directives #3126
• Prep: Update docsy to 0.7.2-dev.0-3f3172a #3128
• Convert all tabpanes - Normalize tabpane shortcode use #3137
• Upstream style changes to Docsy - Tabbed panes: fix styling when not at top-level & other tweaks google/docsy#1671
• Update docsy and drop our local workarounds