accessibility: interactive controls should not be nested

[gorlanova]

Describe the bug

in the collapsible navigation sidebar, group header interactive controls are nested, this is inaccessible for some accessibility tools as the nested button will not always be announced

Reproduction

set a collapsible nav in the sidebar
inspect the tree : the nav group header is "tabindex = 0" and contains a button

Expected behavior

either the header should not have a tabindex, or the header button should simply be an icon (no need for two buttons doing the same thing ?)

System Info

System:
    OS: macOS 14.2.1
    CPU: (8) arm64 Apple M2
    Memory: 393.92 MB / 24.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 20.10.0 - /usr/local/bin/node
    Yarn: 1.22.19 - /opt/homebrew/bin/yarn
    npm: 10.2.5 - /usr/local/bin/npm
    pnpm: 8.9.2 - /opt/homebrew/bin/pnpm
  Browsers:
    Safari: 17.2.1
  npmPackages:
    vitepress: ^1.0.0-rc.31 => 1.0.0-rc.35

Additional context

Validations

• Check if you're on the latest VitePress version.
• Follow our Code of Conduct
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.