Improve and update documentation (Part 1)

* Remove `crossdomain.md` and move its update content into `misc.md`.

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[✔] TOC.md
[✔] crossdomain.md
[ ] css.md
[ ] extend.md
[✔] faq.md
[ ] html.md
[ ] js.md
[ ] misc.md
[ ] usage.md

Cc: @drublic
Merge remote-tracking branch 'origin/update-docs'