Add docs for the LTI Outbound Gem
Add docs for the LTI Outbound Gem

Refs INTEROP-6811
flag=none

Test Plan:
- Verify the new documents are clear and
  error free.

Change-Id: If52b3605de3acf47887b84b3d3ab0bc4d2d0cdfb
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/265973
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Sean Scally <ceb34b930afeb707ad0575cfce0e31dce3c15014@instructure.com>
Reviewed-by: Xander Moffatt <27f70fe194da423604e6e0a63a3de19814736fa9@instructure.com>
QA-Review: Weston Dransfield <e9203059795dc94e28a8831c65eb7869e818825e@instructure.com>
Product-Review: Weston Dransfield <e9203059795dc94e28a8831c65eb7869e818825e@instructure.com>

instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms