Fix acknowledgement for webhooks in non-reply, non-fastack cases

[peterbroadhurst]

Raised from #1025

• Adds better logging
• Ensures we do a DeliveryResponse in the case of a non-reply, non-fastack webhook subscription
• Disabled fastack when reply is configured, as the reply would not be sent otherwise
• (Re)instate the lost events section in the config docs
• Correct the docs to reflect that if you want to retry on non-2xx error codes, you need to set events.webhooks.retry

[codecov-commenter]

Codecov Report

Merging #1072 (65b24c4) into main (cd6a4d9) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 65b24c4 differs from pull request most recent head 96f3e76. Consider uploading reports for the commit 96f3e76 to get more accurate results

@@           Coverage Diff           @@
##             main    #1072   +/-   ##
=======================================
  Coverage   99.99%   99.99%           
=======================================
  Files         307      307           
  Lines       20294    20315   +21     
=======================================
+ Hits        20292    20313   +21     
  Misses          2        2           

Impacted Files	Coverage Δ
internal/events/webhooks/webhooks.go	100.00% <100.00%> (ø)
internal/namespace/manager.go	100.00% <100.00%> (ø)

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[nguyer]

Looks good! Thanks!