Improve reviewer navigation

[mramiropareta]

What changed

• Reorganised the landing page around a reviewer-first “Start here” table.
• Added a concise reviewer guide with three review routes.
• Converted the manuscript output map into a linked index from manuscript items to code and public materials.
• Added directory guides for scripts/ and R/.

Why

The repository already contained the required reproducibility material, but the reviewer path was distributed across several documents. These changes surface the main review routes and reduce the number of clicks needed to trace manuscript claims to code.

Impact

Documentation and navigation only. No analysis code, data, mappings, generated outputs, or privacy rules changed.

Checks

• All local Markdown links resolve.
• python tools/check_public_release.py passed.
• Rscript tools/parse_all_code.R passed.
• git diff --check passed.