Refguide: refresh content of the debugging.adoc section

[simonbasle]

In debugging.adoc there are a number of outdated representation of tracebacks.
Notably:

• In "Reading a Stack Trace in Debug Mode", the fact that now the original stack trace is moved to the end is not reflected
• The term traceback should probably only be used when talking about assembly trace pointing to the original operator use, not the "Error has been observed by the following operator(s)" (fix throughout the debugging section) Let's make sure traceback more clearly refers to whatever additional information reactor adds.
• "Error has been observed by the following operator(s)" is now "Error has been observed at the following site(s)"
• in "the checkpoint alternative": checkpoint rendering doesn't state "light" anymore
• Rendering of named checkpoints with forced stacktrace don't show the name #2819
• In that same section we should probably explain what to expect when having both Hooks.onOperatorDebug() mixed with light checkpoint(String)
• codebase still contains two wordings: traceback (most used 👍) and backtrace (least used 👎) => harmonize on traceback
• ...

[simonbasle]

note the situation has been improved in 3.4.x by #2780 and #2814

[simonbasle]

opening a separate issue for the rendering of checkpoints in default/light/heavy modes: #2819 fixing in #2820