Adds skill to edit and clean up documentation - #631

Adds skill to edit and clean up documentation.

Applies it to duckdb.pyand build-a-plan.md as examples.

🛠️ Changes proposed in this pull request

• Adds a skill for editing documentation
• Includes a script for detecting reading level
• Edits two files to demonstrate
• Adds a repo glossary to build a shared understanding

👀 Guidance to review

Read the docs. Do you agree they're better?

🤖 AI declaration

Script and skill were AI generated then human reviewed and edited. Extensive skill iterations by hand.

✅ Checklist:

• This is the smallest, simplest solution to the problem
• I've read our code standards and this code follows them
• All new code is tested
• I've updated all relevant documentation (select all that apply)
  • API documentation (docstrings and indexes)
  • Tutorials
  • Developer docs