Currently there are two docs workflows available for DTS - the "normal" DPDK
build workflow like:

meson setup my-build
ninja -C my-build doc

And this is totally satisfactory for our purposes.

There is also an alternate docs build process through poetry which is explained
in dts.rst. But, the build result "improvements" for using this are minor and
the downsides associated with promoting this workflow (instead of just
directing people to the standard docs build process) are significant. So, let's
remove this poetry dts docs build explanation from dts.rst.