add titles to all doc files, rewrite setup

[lajbel]

This PR is necessary for replit/kaboomsite#5 working. This PR:

• add titles in every doc/*.md entry
• rewrites doc/setup.md
  • simplify some phrases
  • change the section Using NPM to Setup your own Node.js environment, I think it makes more emphasis that what you use at the end is node.js, and also the existence of more package managers besides npm, npm is only the package manager.
  • clarify that what is shown in the esbuild mini tutorial is what create-kaboom does, I think it promotes (in my opinion) the best way to start kaboom.js projects.

[slmjkdbtl]

Looks great