Improve README structure and add contributor guidance

[mahmoodhamdi]

Summary

Restructure the README around a table of contents and dedicated sections so new
contributors can scan it quickly and find the information they need.

Changes:

• Add a Table of Contents linking to each section
• Convert the project structure overview into a table for easier scanning,
  including the top-level SController base class
• Split the setup into "Quick Start with Docker" and "Manual Setup", with the
  Windows/IIS instructions moved into a subsection of the manual path
• New "Sample Database" section describing the SQL seed files in db/
• New "Contributing" section with PR guidelines (one concern per PR, issue
  reference in the commit message, squash before merge, screenshots for UI
  changes, propose-large-changes-first)
• "Code Style" subsection pointing to php-cs-fixer
• "Content Corrections" subsection redirecting hadith content corrections away
  from this repo and toward the appropriate workflow
• "License" section quoting the BSD-3-Clause field from composer.json

No content was removed without being preserved or expanded elsewhere.

Fixes #157

Test plan

• Renders correctly as Markdown on GitHub
• All TOC anchors resolve to existing headings
• All external links use HTTPS and point to live pages
• No technical claims added that aren't verifiable from the repo

[ahadith]

@fsid could you please review?