link to new lti postMessage docs
link to new lti postMessage docs

refs INTEROP-6636
flag=none

* in 811a119, added this doc file but forgot to link to it in sidebar

test plan:
* dcr web rake doc/api
* under the LTI section, click on window.postMessage
* the postMessage documentation is shown

Change-Id: Ie0e1c01ae9060cca53e39c0eba5e8262b37f3a6f
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/262820
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Weston Dransfield <e9203059795dc94e28a8831c65eb7869e818825e@instructure.com>
Product-Review: Karl Lloyd <4c8060af93345c2cc416ee6aa55bceec4e5cc9d4@instructure.com>
QA-Review: Xander Moffatt <27f70fe194da423604e6e0a63a3de19814736fa9@instructure.com>

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