doc: use basic theme as parent theme instead of default theme
doc: use basic theme as parent theme instead of default theme

mroonga/mroonga,yoku0825/mroonga,mroonga/mroonga,mroonga/mroonga,naoa/mroonga,yoku0825/mroonga,naoa/mroonga,naoa/mroonga,mroonga/mroonga,yoku0825/mroonga,yoku0825/mroonga,naoa/mroonga,naoa/mroonga,yoku0825/mroonga,yoku0825/mroonga,naoa/mroonga