Advanced Concepts/Context Propagation: Add basic content to page

[PikBot]

This page has no content, would it be better to add some filler content using these:

• https://github.com/census-instrumentation/opencensus-specs/blob/master/trace/HTTP.md#propagation
• https://github.com/census-instrumentation/opencensus-specs#context

as a starting point @odeke-em ? Additionally, we could add something like so:

Context propagation is the ability of an application to extract the propagated span context from a variety of spans and then join the trace from that point, across boundaries. In order to propagate span context across process boundaries and RPC calls, inject and extract mechanisms are used.