docs: rewrite intro + FAQ, wire new docs into sidenav

introduction.mdx:
- Adds an opening "What is AdminLTE?" section explaining the project
- Pins Bootstrap version (5.3.8) and Node version (18+ / 22 LTS) in a
  prerequisites table
- Splits installation into four labelled paths (CDN, npm, source build,
  Composer) with audience and setup-time per path
- Adds a "where to next" section linking to Getting Started,
  Customization, Migration
- Drops outdated wording

faq.mdx:
- Replaces the 3-Q stub (which mentioned jQuery ajax / AngularJS / MVC5
  ASP .NET -- all v2/v3 carryovers that don't apply to v4) with
  19 questions across 5 sections: Getting Started, Framework
  Integrations, Customization, Versions and Updates, Licensing,
  Troubleshooting
- Each answer links to the relevant deeper doc page

_sidenav.astro:
- Adds nav entries for the four new docs pages between Installation
  and Recommended Integrations, ordered to follow a learning flow:
  Installation -> Getting Started -> Layout -> Customization ->
  Color Mode -> RTL -> Migration -> Integrations