WR340584, Fix broken search links
The scheduled manual builds are currently producing a manual with broken
search links. The file extension is being set to undefined rather than
the expected .html.
It turns out that these builds are using sphinx-build 3.1.2 and
searchtools.js now expects DOCUMENTATION_OPTIONS.LINK_SUFFIX rather than
DOCUMENTATION_OPTIONS.FILE_SUFFIX which is being produced by layout.
Solution: peg sphinx to the required version. Things can break when a later version becomes available and it has not been tested by us.