book.toml: exclude kde.org

[domsim1]

The userbase.kde.org returns a 403 Forbidden even though the link is valid. I believe this is due to cloudflare bot detection on kde.org. Added kde.org to the excluded list in book.toml.

Before

Checking links with mdbook-linkcheck ...
error: Server returned 403 Forbidden for https://userbase.kde.org/K3b
   ┌─ installation/live-images/prep.md:72:3
   │
72 │ - [K3B](https://userbase.kde.org/K3b)
   │   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Server returned 403 Forbidden for https://userbase.kde.org/K3b

After

Checking for mdbook-linkcheck ... /usr/bin/mdbook-linkcheck
Checking for vmdfmt ... /usr/bin/vmdfmt
Checking links with mdbook-linkcheck ...
Checks completed.

[domsim1]

moved change into #884