added backendapi doc and fixed some links

content/_index.md

content/docs/backendapi/_index.md

content/docs/backendapi/how-to-add-a-new-route.md

...s/backendapi/how-to-setup-your-development-environment.md

content/docs/backendapi/introduction.md

content/docs/contributing/how-to-add-a-new-command.md

content/docs/getting-started/introduction.md

content/docs/getting-started/quick-start.md

content/docs/help/faq.md

content/docs/tutorials/exercise-perform.md