Adding new section checks for 2026 papers

[arfon]

This pull request updates the repository checks to support new paper requirements for submissions from 2026 onwards. It introduces additional section checks and adjusts word count thresholds based on the submission type, as well as improving documentation.

Enhancements for paper section checks and word count:

• The build_repository_history_section method in checks.rb now checks for new required sections (Software Design, Research Impact Statement, and AI usage disclosure) in the paper.md file for submissions without the pre-2026-submission label. It always checks for the Statement of need section.
• Word count thresholds are now dynamic: pre-2026 submissions use the original thresholds (target 1000–1200 words), while 2026+ submissions use expanded thresholds (target 1500–1800 words).

Documentation updates:

• The README.md has been updated to document the new checks for Software Design, Research Impact Statement, and AI usage disclosure sections, clarifying which are required for different submission types.