doc: bump breathe requirement to >=4.23.0

Breathe>=4.23.0 brings in basic xrefitem rendering support and fixes
issues parsing anonymous struct/unions. This version also adds a config
knob for showing or hiding enumerator values, so set it to generate
documentation compatible with previous versions.

Signed-off-by: Fabio Utzig <fabio.utzig@nordicsemi.no>

doc/conf.py

@@ -521,6 +521,7 @@ breathe_domain_by_extension = {
     "c": "c",
 }
 breathe_separate_member_pages = True
+breathe_show_enumvalue_initializer = True
 
 # Qualifiers to a function are causing Sphihx/Breathe to warn about
 # Error when parsing function declaration and more.  This is a list

scripts/requirements-doc.txt

@@ -1,6 +1,6 @@
 # DOC: used to generate docs
 
-breathe>=4.21.0
+breathe>=4.23.0
 docutils>=0.16
 sphinx>=3.2.0,<4.0
 sphinx_rtd_theme