Fix docs build warning for old HMC API books

[andy-maier]

python-zhmcclient/docs/intro.rst:93: WARNING: term not in glossary: 'HMC API 2.11.1'
python-zhmcclient/docs/intro.rst:94: WARNING: term not in glossary: 'HMC API 2.12.0'
python-zhmcclient/docs/intro.rst:95: WARNING: term not in glossary: 'HMC API 2.12.1'

This issue was introduced in PR #1460 (in 1.15 and rolled back into 1.14) which removed the bibliography entries for the old books.

Solution options:

• Add the old books back in to the bibliography, but without links - implemented in PR #1574
• Remove the references to the old books.