docs/reference/api: Document /healthcheck endpoint
docs/reference/api: Document /healthcheck endpoint

This is good. We should tell people about it.

The trailing slash is intentional per `api/server.go`.

Endika/tsuru,guilhermebr/tsuru,mderaldo/tsuru,firebitsbr/tsuru,ArxdSilva/tsuru,kgrvamsi/tsuru,cgvarela/tsuru,4eek/tsuru,4eek/tsuru,tsuru/tsuru,gwmoura/tsuru,tuxmonteiro/tsuru,kgrvamsi/tsuru,gwmoura/tsuru,gciotta/tsuru,guilhermebr/tsuru,gwmoura/tsuru,ggarnier/tsuru,tuxmonteiro/tsuru,gciotta/tsuru,firebitsbr/tsuru,hcxiong/tsuru,cezarsa/tsuru,ggarnier/tsuru,cezarsa/tsuru,tsuru/tsuru,hcxiong/tsuru,kgrvamsi/tsuru,ArxdSilva/tsuru,Endika/tsuru,gciotta/tsuru,cgvarela/tsuru,hcxiong/tsuru,mderaldo/tsuru,tsuru/tsuru,guilhermebr/tsuru,tuxmonteiro/tsuru,ArxdSilva/tsuru,firebitsbr/tsuru,tsuru/tsuru,cezarsa/tsuru,cgvarela/tsuru,guilhermebr/tsuru,mderaldo/tsuru,ArxdSilva/tsuru,cezarsa/tsuru,ggarnier/tsuru,ggarnier/tsuru,4eek/tsuru,Endika/tsuru