docs: prepare project for publication and contribution

[bug-ops]

Summary

• Improve main README with GitHub callouts, better structure, MSRV documentation, and feature flag tables
• Add CONTRIBUTING.md with comprehensive development workflow, commit conventions, and contribution guidelines
• Update CHANGELOG.md with recent Phase 5.1 features (HTTP bindings, podcast support, cargo-make migration)
• Update feedparser-rs-core README with HTTP fetching examples and new features
• Fix markdown lint warnings (duplicate headings)

Changes

README.md

• Add GitHub callouts (> [!IMPORTANT], > [!TIP], > [!NOTE]) for better UX
• Add cargo features table documenting the http feature
• Add workspace structure table with links to all crates
• Document MSRV policy (1.88.0, edition 2024)
• Improve code examples with error handling and URL fetching

CONTRIBUTING.md (new)

• Development prerequisites and setup instructions
• Branch naming conventions
• Commit message format (Conventional Commits)
• Code style guidelines for Rust
• PR process and checklist
• Testing instructions
• Architecture overview

CHANGELOG.md

• Document Phase 5.1 features: HTTP bindings, podcast support
• Document cargo-make migration
• Clean up format

crates/feedparser-rs-core/README.md

• Add HTTP fetching section with code examples
• Add podcast support to features list
• Document how to disable HTTP feature

Test plan

• Verify README renders correctly on GitHub
• Verify all links work
• Verify callouts render properly
• Review CONTRIBUTING.md for completeness

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@           Coverage Diff           @@
##             main      #15   +/-   ##
=======================================
  Coverage   78.80%   78.80%           
=======================================
  Files          27       27           
  Lines        4303     4303           
=======================================
  Hits         3391     3391           
  Misses        912      912           

Flag	Coverage Δ
rust-core	78.80% <ø> (ø)

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