SSR guide: add a chapter about migration from CSR to SSR

[NataliaTepluhina]

By @CyberAP comment:

Since most of the people who want to migrate from CSR to SSR would be having some kind of main.js entry file maybe it would make sense to show this migration path as a step-by-step guide how we extract our current code from main.js and split it into three different files: app.js, entry-client.js and entry-server.js. The current approach is also very good, but I think it's more focused on writing an SSR app from scracth. Maybe there should be a separate guide for migrating from CSR to SSR? What do you think?