doc: fix doc styles

BUILDING.md
  + L122: Missing code-language flag
  + L170: Strong should use `*` as a marker

doc/changelogs/CHANGELOG_V6.md
  + L1494: Don't pad `emphasis` with inner spaces

doc/guides/maintaining-V8.md
  + L3: Don't use multiple top level headings
  + L16: Don't use multiple top level headings
  + L40: Don't use multiple top level headings
  + L124: Don't use multiple top level headings
  + L182: Missing code-language flag
  + L223: Don't use multiple top level headings
  + L288: Don't use multiple top level headings
  + L307: Don't use multiple top level headings

doc/guides/writing-tests.md
  + L322: Missing code-language flag
  + L329: Missing code-language flag

doc/releases.md
  + L299: Missing code-language flag

PR-URL: https://github.com/nodejs/node/pull/13270


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Alexey Orlenko <eaglexrlnk@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>