Add new section in /doc/platforms containing a mainframe overview

[gshriver]

This PR adds a new mainframe section in /docs/platforms and contains a gentle introduction to mainframe systems. This new mainframe section is targeted at readers who may be familiar with cloud-native environments and OpenTelemetry, but who may be unfamiliar with legacy mainframe systems.

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[tiffany76]

Hi @gshriver, thanks for your contribution! Could you sign the CLA? We'll need it signed before we can review your work.