Tabpane shortcode fixes and enhancements (July)

[chalin]

• tabpane: support header as persistence key #1602
• tabpane: use data- attribute instead of class name #1607
  • Rename persistLang() and/or replace it -- https://github.com/google/docsy/pull/1602/files#r1252787856
• Drop persistLang=false from UG pages #1608
  • Address warnings now issued for the user guide
• tabpanes generate duplicate element IDs for tab pane HTML element #1609
• Tabpane: support persistence of all active tabs #1611

[chalin]

@deining - there might be an issue with the new tabpane code, but I'll investigate further once #1607 and #1608 are merged (without #1608 I can't test the new functionality in the User Guide -- it works fine on the OTel website but some issues with the UG).

[chalin]

Interesting, the Hello World tabbed pane in the User Guide stopped working June 5 when we upgraded to Hugo 0.112.3 (so it has nothing to do with recent changes):

• Make docsy theme point to latest version of dependencies #1533 - preview https://deploy-preview-1533--docsydocs.netlify.app/docs/adding-content/shortcodes/#tabbed-panes - tabbed pane still works
• Upgrade Hugo to 0.112.3 and fix en title #1534 - preview https://deploy-preview-1534--docsydocs.netlify.app/docs/adding-content/shortcodes/#tabbed-panes: no longer works. Selecting "C++" doesn't change the code. Selecting "Kotlin" shows two panes.

I'm investigating.

---

I've moved this to a separate issue:

• tabpanes generate duplicate element IDs for tab pane HTML element #1609

[chalin]

Main features added. Moving remaining items to:

• Tabpane shortcode fixes and enhancements (23Q3-Q4) #1641