docs: generate reproducible ids

With this parameter, docbook will generate the same id's when generating the
same docs. This is helpful to get bit-by-bit 'reproducible' output, which
makes auditing the package easier.

Tested both the cmake output and the automake output
11 jobs for docs-generate-reproducible-ids in 8 minutes and 25 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Build
passed #5447507
build:cmake

00:03:18

passed #5447504
build:debug

00:07:25

manual #5447509
allowed to fail manual
build:i686-w64-mingw32-cmake
passed #5447508
build:i686-w64-mingw32-debug

00:05:02

manual #5447506
allowed to fail manual
build:legacy
passed #5447503
build:production

00:04:27

manual #5447505
allowed to fail manual
build:reduced
manual #5447512
allowed to fail manual
build:stretch
passed #5447510
build:x86_64-w64-mingw32

00:04:40

passed #5447511
build:x86_64-w64-mingw32-cmake-debug

00:08:24

manual #5447513
allowed to fail manual
build:xenial