Improve PEcAn.SIPNET documentation and connect to PEcAn book

[Eshaan-byte]

This PR addresses #3703 by consolidating and improving SIPNET model documentation across the PEcAn project, completing Task 1 from the issue.

Changes Made

1. Enhanced PEcAn.SIPNET README (models/sipnet/README.md)

• Added comprehensive "What is SIPNET?" section with key features
• Added "Using SIPNET via PEcAn" section explaining package integration
• Included proper documentation links to SIPNET docs, package docs, and tutorials
• Replaced generic placeholder text with meaningful content
• Separated installation instructions for package vs. model executable

2. Updated PEcAn Book SIPNET Page (book_source/03_topical_pages/05_models/sipnet.Rmd)

• Filled in missing model metadata (homepage, source code, license, authors)
• Replaced "Introduction about model" template text with actual model description
• Added links to standalone SIPNET documentation and package website

3. Modernized Installation Instructions (install_SIPNET.Rmd)

• Replaced outdated tarball download instructions with current repository-based method
• Added links to official SIPNET installation guide
• Included deprecation notice for old installation methods

4. Updated Project Files

• Added entries to NEWS.md (PEcAn.SIPNET package)
• Added entry to CHANGELOG.md (project-wide)

Testing

The documentation changes have been reviewed for:

• Accurate links to external resources
• Consistency with standalone SIPNET documentation
• Proper markdown formatting
• Clear and helpful content for users

Next Steps

This PR completes Task 1 from #3703. Remaining tasks include:

• Task 2: Further refinement of book page content
• Task 3: Validating documentation builds correctly
• Task 4: Documenting the process for other model couplers

I'm happy to address any feedback and make additional improvements as needed.