docs: Fix read-the-docs

[martin-belanger]

Allow Read-the-Docs web site to be able to build the docs.

@igaw - I made a change to the meson.build files to allow building the documentation without building anything else. This will allow the Read-the-Docs website to only build the documentation w/o anything else.

Note that the Read-the-Docs libnvme project (https://app.readthedocs.org/projects/libnvme/) needs to be updated to point to linux-nvme/nvme-cli instead of linux-nvme/libnvme.

[igaw]

Thanks!

Ideally we get the libnvme read the docs configuration so it has the docs for the 3.x and the 1.x version at the same time.