mark overrides as a subtopic in api documentation
mark overrides as a subtopic in api documentation

fixes CNVS-5486

test plan:
- generate api docs and navigate to them
- go to assignments in the left nav
- the index at the top of the page should be split into two sections

Change-Id: I1680d19508cf84f735e6e257fdfbcaf2affcae21
Reviewed-on: https://gerrit.instructure.com/19998
Reviewed-by: Stanley Stuart <2b43fb8b7a234825d50dd49ce7892d78a59da8f3@instructure.com>
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
QA-Review: Myller de Araujo <e5c7c5e1b7f2ce5e9870cc98ec52b20b3006cb48@instructure.com>
Product-Review: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>

eCNAP/CANVAS,Unow/canvas-lms,jay3126/canvas-lms,antoniuslin/canvas-lms,leftplusrightllc/canvas-lms,utkarshx/canvas-lms,redconfetti/canvas-lms,arrivu/qibla-ecampus,efrj/canvas-lms,arrivu/hoodemo,wye0220/shindig-canvas,kyroskoh/canvas-lms,utkarshx/canvas-lms,va7map/canvas-lms,Rvor/canvas-lms,AndranikMarkosyan/lms,Rvor/canvas-lms,narigone/canvas,sfu/canvas-lms,Rvor/canvas-lms,antoniuslin/canvas-lms,juoni/canvas-lms,greyhwndz/canvas-lms,arrivu/hoodemo,usmschuck/canvas,efrj/canvas-lms,landonwilkins/canvas-lms,SwinburneOnline/canvas-lms,arrivu/arrivu-lms,dgynn/canvas-lms,fronteerio/canvas-lms,sdeleon28/canvas-lms,snehakachroo1/LSI_replica,djbender/canvas-lms,grahamb/canvas-lms,matematikk-mooc/canvas-lms,matematikk-mooc/canvas-lms,antoniuslin/canvas-lms,skmezanul/canvas-lms,instructure/canvas-lms,redconfetti/canvas-lms,sfu/canvas-lms,narigone/canvas,narigone/canvas,Unow/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,whalejasmine/canvas-lms,tgroshon/canvas-lms,samuels410/greatlakes-lms,Jyaasa/canvas-lms,ShreniiNepal/shrenii_lmp,sdeleon28/canvas-lms,whalejasmine/canvas-lms,ShreniiNepal/shrenii_lmp,roxolan/canvas-lms,utkarshx/canvas-lms,snehakachroo1/LSI_replica,kyroskoh/canvas-lms,greyhwndz/canvas-lms,AndranikMarkosyan/lms,wrdsb/canvas-lms,eCNAP/CANVAS,snehakachroo1/LSI_replica,venturehive/canvas-lms,eCNAP/CANVAS,jay3126/canvas-lms,loganrun/pincan,tgroshon/canvas-lms,SwinburneOnline/canvas-lms,grahamb/canvas-lms,HotChalk/canvas-lms,venturehive/canvas-lms,gbleydon/canvas-lms,efrj/canvas-lms,Unow/ins,SwinburneOnline/canvas-lms,jay3126/canvas-lms,sfu/canvas-lms,va7map/canvas-lms,dgynn/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,usmschuck/canvas,wrdsb/canvas-lms,roxolan/canvas-lms,leftplusrightllc/canvas-lms,va7map/canvas-lms,skmezanul/canvas-lms,dgynn/canvas-lms,grahamb/canvas-lms,tgroshon/canvas-lms,gbleydon/canvas-lms,Jyaasa/canvas-lms,djbender/canvas-lms,grahamb/canvas-lms,arrivu/hoodemo,instructure/canvas-lms,antoniuslin/canvas-lms,matematikk-mooc/canvas-lms,wrdsb/canvas-lms,arrivu/arrivu-lms,wimemx/Canvas,shidao-fm/canvas-lms,matematikk-mooc/canvas-lms,wye0220/shindig-canvas,HotChalk/canvas-lms,snehakachroo1/LSI_replica,instructure/canvas-lms,sdeleon28/canvas-lms,sfu/canvas-lms,kyroskoh/canvas-lms,greyhwndz/canvas-lms,grahamb/canvas-lms,utkarshx/canvas-lms,loganrun/pincan,shidao-fm/canvas-lms,leftplusrightllc/canvas-lms,usmschuck/canvas,roxolan/canvas-lms,venturehive/canvas-lms,sfu/canvas-lms,arrivu/qibla-ecampus,Unow/ins,arrivu/qibla-ecampus,juoni/canvas-lms,juoni/canvas-lms,Unow/ins,tgroshon/canvas-lms,merichar/canvas-lms,venturehive/canvas-lms,wrdsb/canvas-lms,fronteerio/canvas-lms,juoni/canvas-lms,sdeleon28/canvas-lms,greyhwndz/canvas-lms,gbleydon/canvas-lms,landonwilkins/canvas-lms,loganrun/pincan,kyroskoh/canvas-lms,va7map/canvas-lms,whalejasmine/canvas-lms,arrivu/jigsaw-lms,HotChalk/canvas-lms,jay3126/canvas-lms,djbender/canvas-lms,fronteerio/canvas-lms,Jyaasa/canvas-lms,AndranikMarkosyan/lms,redconfetti/canvas-lms,whalejasmine/canvas-lms,samuels410/greatlakes-lms,instructure/canvas-lms,Unow/canvas-lms,skmezanul/canvas-lms,leftplusrightllc/canvas-lms,usmschuck/canvas,instructure/canvas-lms,wimemx/Canvas,SwinburneOnline/canvas-lms,Jyaasa/canvas-lms,samuels410/greatlakes-lms,HotChalk/canvas-lms,merichar/canvas-lms,ShreniiNepal/shrenii_lmp,landonwilkins/canvas-lms,Rvor/canvas-lms,dgynn/canvas-lms,instructure/canvas-lms,redconfetti/canvas-lms,landonwilkins/canvas-lms,skmezanul/canvas-lms,samuels410/greatlakes-lms,eCNAP/CANVAS,arrivu/jigsaw-lms,wimemx/Canvas,fronteerio/canvas-lms,arrivu/arrivu-lms,ShreniiNepal/shrenii_lmp,merichar/canvas-lms,shidao-fm/canvas-lms,efrj/canvas-lms,wye0220/shindig-canvas,roxolan/canvas-lms,shidao-fm/canvas-lms,narigone/canvas,arrivu/jigsaw-lms,AndranikMarkosyan/lms