doc: fix doc styles

CONTRIBUTING.md
  + L857: Unused definition
  + L861: Unused definition
  + L863: Unused definition

doc/api/assert.md
  + L719: Unused definition

doc/api/async_hooks.md
  + L460: Missing code-language flag

doc/api/child_process.md
  + L1362: Unused definition

doc/api/dns.md
  + L674: Unused definition

doc/api/esm.md
  + L178: Missing code-language flag

doc/api/http.md
  + L1868: Unused definition
  + L1887: Unused definition
  + L1888: Unused definition
  + L1889: Unused definition
  + L1916: Unused definition
  + L1917: Unused definition

doc/api/https.md
  + L260: Unused definition

doc/api/os.md
  + L1226: Unused definition

doc/api/process.md
  + L1888: Unused definition

doc/api/stream.md
  + L2227: Definitions with the same identifier

doc/guides/writing-and-running-benchmarks.md
  + L1: Missing newline character at end of file

Refs: https://github.com/nodejs/node/pull/12756
PR-URL: https://github.com/nodejs/node/pull/16385


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>