[zh] Strip away embedded HTML en fragments

[chalin]

Follow through of:

• [zh] Localize context-propagation.md #4536 (comment)
• [i18n] Propose way to associate translations to specific commit of default lang page #4582

For the record, here are the commands I used:

find content/zh -name '*.md' -exec perl -i -0777 -pe 's/<!--.*?-->//gs' {} \;
find content/zh -name '*.md' -print0 | xargs -0 perl -i -0777 -pe 's/\n{2,}/\n\n/gs'