Improve FAQ and examples at guides.

[EugenKon]

The error message

It is Ok to have this message in the logs:

[2025-09-22 18:54:09.46063] [9637] [trace] [7eA72gm_vxF_] Nothing has been rendered, expecting delayed response (see FAQ for more)

But FAQ:

https://docs.mojolicious.org/Mojolicious/Guides/FAQ#What-does-the-error-Maximum-start-line-size-exceeded-mean

Mojolicious has been designed from the ground up for non-blocking I/O and event loops.
So when a new request comes in and no response is generated right away, it will assume
that this was intentional and returns control to the web server, which can then handle
other requests while waiting for events such as timers to finally generate a response.

actually useless, because It gives no clue how to resolve the problem.

I found this SO response very helpful: https://stackoverflow.com/a/76002665/4632019

It whould be nice to add into FAQ link to the render_later sub https://docs.mojolicious.org/Mojolicious/Controller#render_later

and update examples:
https://docs.mojolicious.org/Mojolicious/Guides/Tutorial#User-agent
https://docs.mojolicious.org/Mojolicious/Guides/Cookbook#Synchronizing-non-blocking-operations

And probably this one:
https://docs.mojolicious.org/Mojolicious/Guides/Cookbook#Proxy

It would be nice to have an example in the "Rendering guide" with non-blocking render_later example
in addition to this description of differences: https://docs.mojolicious.org/Mojolicious/Guides/Rendering#Automatic-rendering