feat: add configurable right sidebar - #2772

Summary

Adds a sidebarPosition configuration option so the default theme can render its sidebar on either side of the page.

• keeps 'left' as the default, preserving existing behavior;
• applies the right-side layout to the sidebar, toggle, content, and navbar;
• handles the mobile overlay and toggle target from the right edge;
• documents the new option;
• adds desktop and mobile end-to-end coverage.

Related issue, if any:

Closes #1282

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

For any code change,

• Related documentation has been updated, if needed
• Related tests have been added or updated, if needed

Does this PR introduce a breaking change?

• Yes
• No

Validation

• npm ci / production build
• npm run lint
• npm run typecheck
• npm run test:jest — 13 suites, 111 tests, 46 snapshots
• npx playwright test test/e2e/configuration.test.js --grep sidebarPosition — 6 desktop/mobile cases

Tested in the following browsers:

• Chrome (Playwright Chromium)
• Firefox (Playwright Firefox)
• Safari (Playwright WebKit)
• Edge