prevent page break between the reference tag and reference entry in PDF

[alicerusso]

Description

Example in screenshot below - page 25/26 break in https://www.rfc-editor.org/v3test/test9957.pdf
(source: https://www.rfc-editor.org/v3test/test9957.xml)
h/t @mferguson-rpc for flagging this example.

(This was also requested in a closed issue: #463)

Code of Conduct

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