Document MVP of Blackboard LTI / REST integration

[cbeach47]

Need to document the configuration from the Bb LTI / REST integration effort ( https://github.com/openequella/openEQUELLA/labels/Integration%20-%20Bb%20LTI%2FREST%20MVP ).

It'll be in 3 phases - LTI MVP, Migration building block, and REST MVP

[cbeach47]

LTI and Migration B2 is now released and available to the community, as well as these docs. Still need to complete the REST part.

[cbeach47]

Note:
openequella/openEQUELLA#1197 (discussion on fixed user)
openequella/openEQUELLA#844 (REST application ID)

[cbeach47]

Note:
include a comment about when tokens expire. Also note Bb tokens have a lifespan of 1 hour.