When running meson install such as:
meson setup ci_docs -Denable_docs=true -Dexamples=all -Dplatform=generic
-Ddefault_library=shared -Dbuildtype=debug -Dcheck_includes=true
-Denable_deprecated_libs=* -Dlibdir=lib -Dwerror=true
DESTDIR=/home/jlinkes/workspace/dpdk/build-install meson install -C ci_docs

The index.html file from API docs get overwritten by the guides index.html:
Installing /home/jlinkes/workspace/dpdk/dpdk/ci_docs/doc/api/html/index.html to
/home/jlinkes/workspace/dpdk/build-install/usr/local/share/doc/dpdk/html
...
Installing /home/jlinkes/workspace/dpdk/dpdk/ci_docs/doc/guides/html/index.html
to /home/jlinkes/workspace/dpdk/build-install/usr/local/share/doc/dpdk/html

The resulting installed API docs are thus missing their index.html page.