Remove dead urls from docs

[pp-mo]

On re-implementing the broken-links checking,
found that several of the 'excluded' links appear to be permananently "gone" web resources.

Apart from those in old whats-new pages, these should be re-thought + replaced.
( though some might be locatable via wayback ? )

See lines marked "todo: remove" in the file ./lycheeignore :
( these are all broken links, which the check is currently skipping over )

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