Ember docs on 023

[valencik]

This PR updates all the mdoc checked docs to use ember instead of blaze.

This is a continuation of http4s#4932 and http4s#4279.
Everything is rebased on latest series/0.23, but I can change the target branch if desired.

I have introduced three changes that are worth calling out as they may be controversial.

Sleeping after ember server start

Starting the ember server in mdoc now looks like:

val startAndSleep = server.use(_ => IO.never).start <* IO.sleep(2.seconds)
val fiber = startAndSleep.unsafeRunSync()

Without an IO.sleep, any attempt to connect to the ember server would fail with a java.net.ConnectException: Connection refused as the request would be attempted before the ember server was actually bound to the address.

Adding logback to docs/mdoc

This was absolutely crucial for me to figure out what was going on with the connect refusal error above.
With logging enable I could see the ConnectException and then a second later the ember server log:

[info] [io-compute-15] INFO  o.h.e.s.EmberServerBuilderCompanionPlatform - Ember-Server service bound to address: [::]:8080

I think having the logs is worthwhile if someone is running mdoc, it gives us slightly more visibility into what is going on.
I stole the logback.xml from the gitter8 template.

Forking mdoc

This is really just the workaround described in scalameta/mdoc#613 to avoid terminating sbt when mdoc fails.

Ditching nest mdoc modifier

I've removed nest mdoc modifiers in client.md because I found they only added confusion.
I've held off removing nest from the other docs as it turned out to not be necessary, but I feel like it's still worthwhile.
I don't think it buys us much.