docs(vitepress): pending tasks

[userquin]

To support dark/light theme and add some new cool features using @vue/theme

These tasks have been moved from #573 since @vue/theme has breaking changes for vitest docs:

• [x] add @vue/theme: dark/light switcher, scrollable menu spy support, fixed navbar on scroll, previous and next page on footer, social icons...
• [x] add algolia search: maybe including @vue/theme will work adding the key on the configuration
• [ ] a11y : review color contrast
• [ ] fix LCP and CLS
• [ ] fix PWA: it is broken