fix: sidebar toggle behavior

[AdamJ]

Description of changes

Changed the behavior of the sidebar toggle to automatically close when another is opened - this matches the behavior found with the Bootstrap accordion component.

Fixes #24

PR checklist

Please check the boxes that apply before requesting review.

• I ran the site locally (pnpm show) and verified the changed pages
• I ran the production build (pnpm css && pnpm build) and checked docs/ output
• I updated or copied vendor assets using the package.json copy scripts if required (pnpm copy-css|copy-js|copy-fa|copy-cas)
• If this change depends on caswow.library, I documented whether it was linked with npm link or included in CI
• I verified there are no uncaught exceptions in the browser console (common: dialog script in base.njk)
• I added notes in this PR about any assumptions made (missing vendor files, local links, env variables)