* Update README.md

* Update README_en-US.md

* external alternative search

* external alternative search

* external alternative search

* external alternative search

* docs: update readme.md

* feat: add assist_search

* docs: add doc for assist_search & update some others

* docs: add doc for hexo-submit-urls-to-search-engine
Co-authored-by: liuyib <1656081615@qq.com>