doc: update synopsis, toc, other nits

[Fishrock123]

Checklist

• tests and code linting passes
• documentation is changed or added
• the commit message follows commit guidelines

Affected core subsystem(s)

doc

Description of change

There are a few things here:

• Updated the Example to be more modern and better reflect the example in https://nodejs.org/en/about/
• Renamed Synopsis to Usage & Example
• Node.js(1) in the TOC is now Node.js since the former does not make sense in any context
• The TOC is now subdivided: general about, subsystems, extra links
• Added a toc / sidebar link to the repo/issue tracker and also the mailing list

cc @nodejs/documentation