Add support for nesting and collapsing sidebar groups

[delucis]

Description (required)

Updates our utility function mapping our nav.ts files to Starlight’s sidebar config shape to support nesting and collapsing groups.

Syntax

• Nesting/collapsing is controlled from src/i18n/en/nav.ts. Other languages continue to work the same way, providing just the key–value mapping of translations.

• Each heading (entries with header: true) can be marked as a nested group by adding nested: true.

• Each heading (entries with header: true) can be marked as collapsed by default by adding collapsed: true.

• I took a few shortcuts to keep this simple, which means nested groups have to be at the end of their containing group, i.e. the following is possible:

  Parent group     ▼
  |   Link 1
  |   Link 2
  |
  |   Sub group    ▼
  |   |   Link 3
  |   |   Link 4
  |
  |   Other group  ▼
  |   |   Link 5
  |
  Other group      ▼
  |   Link 6
  

  But this is not:

  Parent group     ▼
  |   Link 1
  |   Link 2
  |
  |   Sub group    ▼
  |   |   Link 3
  |   |   Link 4
  |
  |   Link 5
  |   Link 6
  |
  Other group      ▼
  |   Link 7
  

  Based on what I’ve seen so far, that seemed to align with intended usage, but let me know if it’s not!

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Name	Status	Preview	Comments	Updated (UTC)
docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 8, 2024 2:38pm

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Name	Status	Preview	Comments	Updated (UTC)
docs-i18n	⬜️ Ignored (Inspect)			May 8, 2024 2:38pm

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Tracked Files

Locale	File	Note
en	nav.ts	Source changed, localizations will be marked as outdated.

Warnings reference

Icon	Description
🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

[TheOtterlord]

LGTM! Managed to test implement the sidebar redesign with these changes