Merge the Platform + Guide selector

[stephanie-anderson]

As a first step of getting closer to our redesign, we wanna merge the current platform dropdown with the guide sidebar (on the right side), and move the new (combined) dropdown to the left sidebar.

The design changes are currently documented in Figma. But the new platform/guide dropdown will move into the left sidebar, instead of the top navigation (as shown in Figma). The design / functionality of the dropdown will be very similar to this:

[getsantry[bot]]

Routing to @getsentry/product-owners-docs for triage ⏲️