use nimlint to enforce best practices

[ringabout]

Ref RFC timotheecour#415 and https://github.com/nim-compiler-dev/nimlint

Before this PR

D:\QQPCmgr\Desktop\Nim>nimlint -i=lib/pure/distros.nim -o:out.nim --verbose=true
[lint] hintCodeBlocks: D:\QQPCmgr\Desktop\Nim\lib\pure\distros.nim(21)
code blocks => runnableExamples

[lint] hintBackticks: D:\QQPCmgr\Desktop\Nim\lib\pure\distros.nim(204)
double backticks => single backtick

After this PR

D:\QQPCmgr\Desktop\Nim>nimlint -i=lib/pure/distros.nim -o:out.nim --verbose=true

[timotheecour]

if defined(nimEnableExtraExamples) and detectOs(Ubuntu):
   ...

that way it won't actually run the install commands on ubuntu simply because you ran nim doc

and if instead of when so that nim doc checks that the disabled block compiles

[timotheecour]

since this relies on nimlint automation, why not try it on a larger set of files, eg lib/pure/* or even lib/*, and, say, just do it for 1 linter check (double backticks => single backticks) (single lint category makes reviewing easier)

and see whether there are adjustments required (stress test for nimlint)