wiki/Development: rework some presentation and fix typos

This is what came out after reviewing the page from top to bottom as requested in lts-extra-tasks#95 (closed). Most of the suggestions are only cosmetic presentation changes, there is only one cosmetic writing change (58b605ac) which I tend to avoid but seemed like a general improvement in this case. I also added a made up git diff to explain what "claim the issue in security-tracker" or "add your name" actually means.