feature: synchronize code tab changes across page through updated docsy theme

[grayside]

This PR updates to HEAD in docsy.

We are targeting HEAD as opposed to a tagged release because we are using git submodule management. Switching to the new hugo modules subsystem will allow easier management of stable releases.

There have been a number of docsy releases since this repository first implemented it as hugo theme, of most interest here is that tabpanes with automatic code highlighting now "swap" in sync, across the whole page. Tabpanes where code highlighting is disabled are not included, it's unclear but I suspect this is a bug.

• Remove: Custom content-tab shortcodes, which were broken in this update and deprecated by new functionality in the built-in tab shortcode.
• Keep: Customized alert shortcode because markup bug remains
• Keep: Custom include shortcode because the new built-in shortcode for file inclusion has some weird interactions with tab, and sorting that out is it's own task.
• Added: npm install in top-level directory also runs npm install in themes/docsy directory. Identifying how to simplify that noted below.

Follow-up work:

• The recommended approach to install hugo themes (and the preference from docsy) is to use the hugo modules system
• docsy now has new/different dependencies, that expect to be installed from the theme instead of from a parent-level package.json. It's unclear if the npm dependencies currently being installed at the top-level are still required.

Once this PR is merged I will file a follow-up issue.

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🔎 Preview at https://GoogleCloudPlatform.github.io/samples-style-guide/previews/PR-133/

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🔎 Preview at https://GoogleCloudPlatform.github.io/samples-style-guide/previews/PR-133/

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🔎 Preview at https://GoogleCloudPlatform.github.io/samples-style-guide/previews/PR-133/

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🧨 Preview deployments removed.