doc: Pin pip to <25.3 for RTD as a workaround for pybind

doc: Pin pip to <25.3 for RTD as a workaround for pybind

Readthedocs now uses pip 25.3 by default which requires PEP 517.
src/pybind/* does not provide pyproject.toml files for PEP 517.
For an immediate workaround to allow RTD builds to succeed, pin pip
version to earlier than 25.3.

Details for pybind in https://tracker.ceph.com/issues/73645

Signed-off-by: Ville Ojamo <14869000+bluikko@users.noreply.github.com>
(cherry picked from commit a65c46c24952a4f5778c0b68b0e7cf26df32757a)

 Conflicts:
	admin/doc-read-the-docs.txt
 - kept only the pip pinning