Update docs to reflect http plugin merge

[miniduikboot]

Description

Now that we are retiring the Impostor.Http plugin, we still need to do four things:

• Update the documentation
• Remove the builtin docker plugin
• Archive Impostor.Http
• Update Reactor.Impostor
• Write upgrade instructions from 1.8.4

This PR tackles the first 2 items. Upgrade instructions are here:

Impostor 1.9.0 now includes the Impostor.Http plugin. This
If you installed using Docker, you don't have to do anything. The new image no longer comes with the Impostor.Http plugin preinstalled.
If you've used Impostor.Http before, remove it from your plugins folder and put your HttpServer configuration in config.json. Check Server-configuration.md and config.json for reference on how to do this.
If you haven't used Impostor.Http before, install ASP.Net Core 7 and set up the Http Server as detailed in HttpServer.md
Also, if you use Reactor.Impostor, deinstall Reactor.Impostor.Http and update to the latest version of Reactor.Impostor.

Merge after #553 and close to the release of 1.9